PDF Glossary: Terms & Definitions
A comprehensive reference of PDF terminology. Whether you are compressing files, converting formats, or securing documents, this glossary explains the concepts behind every feature.
A
AES (Advanced Encryption Standard)
A symmetric encryption algorithm widely used to secure PDF documents. AES-128 and AES-256 are common variants, with AES-256 providing stronger protection.
Modern PDF encryption tools use AES-256 by default for maximum security when password-protecting sensitive documents.
Try our Protect PDF tool →Annotation
A markup element added to a PDF page, such as highlights, sticky notes, text comments, stamps, or drawing shapes. Annotations exist as a layer on top of the page content.
Reviewers commonly add annotations to mark up contracts or academic papers before sending them back for revision.
Try our Edit PDF tool →Archival Format (PDF/A)
An ISO-standardized subset of PDF designed for long-term digital preservation. PDF/A files embed all fonts, disable encryption, and avoid external dependencies to ensure the document can be rendered identically decades later.
Government agencies and libraries often require documents in PDF/A format to guarantee they remain readable without specialized software.
Try our PDF/A Converter tool →B
Batch Processing
The ability to apply an operation (such as compression, conversion, or watermarking) to multiple PDF files in a single session rather than one at a time.
Batch processing saves significant time when you need to compress an entire folder of scanned receipts before uploading them.
Try our Compress PDF tool →Bookmark
A navigational shortcut embedded in a PDF that links to a specific page or section. Bookmarks appear in a sidebar panel and function like a clickable table of contents.
Long reports and manuals use bookmarks so readers can jump directly to the chapter they need without scrolling.
Try our Organize PDF tool →C
Compression
The process of reducing a PDF file's size by optimizing images, removing redundant data, and applying encoding algorithms. Compression can be lossless (no quality loss) or lossy (slight quality reduction for smaller size).
Compressing a 15 MB scanned document down to 500 KB makes it small enough to attach to an email or upload to a government portal.
Try our Compress PDF tool →Crop Box
A PDF page boundary that defines the visible region of the page when displayed or printed. Content outside the crop box is hidden but not deleted from the file.
Cropping a PDF removes unwanted margins or borders, such as trimming white space around a scanned page to focus on the content.
Try our Crop PDF tool →Cross-Reference Table (XRef)
An internal PDF structure that maps each object in the file to its byte offset, enabling random access to any page or resource without reading the entire file sequentially.
A corrupted cross-reference table is one of the most common causes of "unable to open PDF" errors in viewers.
Try our Repair PDF tool →D
Digital Signature
A cryptographic mechanism embedded in a PDF that verifies the identity of the signer and ensures the document has not been altered after signing.
Contracts, NDAs, and legal agreements increasingly use digital signatures as a legally binding alternative to wet-ink signatures.
Try our Sign PDF tool →Document Properties
Metadata fields stored inside a PDF file, including the title, author, subject, keywords, creation date, and the software used to create it.
Setting proper document properties helps with file organization and improves how the PDF appears in search results and file listings.
DPI (Dots Per Inch)
A measure of image resolution indicating how many dots fit in one linear inch. Higher DPI means finer detail but larger file sizes.
Scanned documents at 300 DPI strike a good balance between readability and file size, while 600 DPI is typically reserved for archival-quality scans.
E
Embedded Fonts
Font data stored directly inside the PDF file so the document renders correctly on any device, even if the viewer does not have the original fonts installed.
Embedding fonts prevents the common issue where a PDF looks different on another computer because a substituted font changes the spacing or appearance of text.
Encryption
The process of encoding a PDF's content so it can only be read with the correct password or certificate. PDF encryption protects against unauthorized viewing, editing, and printing.
Encrypting financial statements or medical records before sharing them ensures only the intended recipient can access the contents.
Try our Protect PDF tool →EXIF Data
Exchangeable Image File Format metadata embedded in images, containing information like camera settings, GPS coordinates, date taken, and device model.
When converting photos to PDF, EXIF data may carry GPS location information that you should strip for privacy before sharing.
Try our JPG to PDF tool →F
Flattening
The process of merging all interactive layers of a PDF (annotations, form fields, signatures) into a single static image layer, making them non-editable.
Flattening a filled-out form before submitting it ensures the data cannot be altered by the recipient and reduces compatibility issues across different PDF viewers.
Font Subsetting
The practice of embedding only the specific characters (glyphs) used in a document rather than the entire font file, reducing the PDF's size.
A document using only 50 characters from a 3,000-glyph font can be significantly smaller when font subsetting is applied.
Form Fields
Interactive elements in a PDF such as text inputs, checkboxes, radio buttons, and dropdown menus that allow users to fill in data directly within the document.
Tax forms and application documents often use PDF form fields so users can type their information and submit the completed form digitally.
Try our Edit PDF tool →G
Grayscale Conversion
The process of converting all color content in a PDF (images and vector graphics) to shades of gray, which reduces file size and is suitable for black-and-white printing.
Converting a color brochure to grayscale before printing on a monochrome printer saves ink and can reduce the file size by up to 30 percent.
Try our Compress PDF tool →H
Header/Footer
Repeated text or graphics that appear at the top (header) or bottom (footer) of every page in a PDF, commonly used for page numbers, document titles, dates, or company logos.
Adding page numbers as footers to a merged document helps readers navigate the combined file and reference specific pages.
Try our Edit PDF tool →Hyperlink
A clickable link embedded in a PDF that navigates to a web URL, an email address, or another location within the same document.
Interactive reports and proposals use hyperlinks to let readers click through to source materials, appendices, or related web pages.
I
Image Compression
The reduction of image data size within a PDF using algorithms like JPEG, JPEG2000, or JBIG2. This is typically the largest contributor to PDF file size reduction.
Since images often account for over 90 percent of a scanned PDF's file size, image compression is the most effective way to shrink these documents.
Try our Compress PDF tool →Interactive Form
A PDF containing fillable form fields that users can complete digitally, including text fields, checkboxes, signature areas, and submit buttons.
Organizations distribute interactive PDF forms to collect standardized information from applicants without requiring specialized software.
Try our Edit PDF tool →J
JBIG2
A compression standard optimized for bi-level (black and white) images, commonly used for scanned text pages in PDFs. It achieves much higher compression ratios than JPEG for text-heavy documents.
JBIG2 compression can reduce a 100-page scanned text document to a fraction of its JPEG-compressed size while keeping text perfectly sharp.
JPEG
A lossy image compression format widely used inside PDFs to store photographic images. JPEG reduces file size by discarding visual information that is less perceptible to the human eye.
Most PDF compressors re-encode embedded images as optimized JPEGs to dramatically reduce the overall file size.
Try our PDF to JPG tool →K
Kerning
The adjustment of horizontal spacing between specific pairs of characters in a font to improve visual consistency and readability.
Proper kerning in a PDF ensures that letter pairs like "AV" or "To" appear evenly spaced, maintaining the professional appearance of typeset text.
L
Linearization (Fast Web View)
A PDF optimization that rearranges the file structure so the first page can be displayed immediately while the rest of the document continues downloading in the background.
Linearized PDFs are ideal for web-hosted documents because users see content instantly rather than waiting for the entire file to download.
Lossless Compression
A compression method that reduces file size without discarding any data, meaning the original content can be perfectly reconstructed. Common lossless algorithms in PDFs include Flate (zlib) and LZW.
Lossless compression is preferred for text-heavy PDFs and technical drawings where every detail must be preserved exactly.
Try our Compress PDF tool →Lossy Compression
A compression method that achieves smaller file sizes by permanently removing some data, typically visual details in images that are difficult for the human eye to notice.
Lossy compression at moderate quality settings can shrink a photo-heavy PDF by 80 to 95 percent with minimal perceptible quality loss.
Try our Compress PDF tool →M
Merge
The process of combining two or more separate PDF files into a single document, preserving the pages and content of each source file in sequence.
Merging individual chapter files into one combined PDF is common when assembling reports, portfolios, or application packages.
Try our Merge PDF tool →Metadata
Information stored within a PDF file that describes the document itself, including title, author, creation date, keywords, and modification history. Metadata is not visible on the pages but can be read by search engines and file managers.
Cleaning metadata before sharing a PDF removes potentially sensitive information like the original author name or editing software version.
MozJPEG
An open-source JPEG encoder developed by Mozilla that produces significantly smaller JPEG files at the same visual quality compared to standard encoders.
PDF Smaller uses MozJPEG via WebAssembly to achieve superior compression ratios entirely within the browser, without sending files to a server.
Try our Compress PDF tool →N
N-up Printing
A layout technique that arranges multiple PDF pages onto a single physical sheet (for example, 2-up places two pages side by side, 4-up arranges four pages in a grid).
N-up printing is useful for creating handouts, saving paper, or reviewing multi-page documents in a compact format.
O
OCR (Optical Character Recognition)
Technology that analyzes images of text (such as scanned pages or photographs) and converts them into machine-readable, searchable, and selectable text.
Running OCR on a scanned contract turns it from a flat image into a searchable document, allowing you to find specific clauses with Ctrl+F.
Try our PDF OCR tool →OPFS (Origin Private File System)
A browser API that provides a sandboxed, high-performance file system for web applications. It enables efficient temporary storage of large files during client-side processing.
PDF Smaller uses OPFS to handle large files during compression without consuming excessive browser memory, enabling the processing of files over 200 MB.
Owner Password
A PDF password that controls permission restrictions such as printing, copying text, and editing. Unlike the user password, the owner password does not prevent opening the document.
Setting an owner password allows you to distribute a PDF that anyone can read but cannot print or copy text from.
Try our Protect PDF tool →P
Page Size
The dimensions of a PDF page, defined in points (1 point = 1/72 inch). Common sizes include Letter (8.5 x 11 inches), A4 (210 x 297 mm), and Legal (8.5 x 14 inches).
Resizing PDF pages to A4 before printing ensures the content fits correctly on standard international paper without unexpected cropping.
Try our Resize PDF tool →PDF (Portable Document Format)
A file format created by Adobe in 1993 (now an open ISO standard) designed to present documents consistently across all devices, operating systems, and software applications.
PDF has become the universal format for sharing documents because a PDF looks the same whether viewed on Windows, Mac, a phone, or printed on paper.
PDF/A
An ISO-standardized version of PDF specifically designed for long-term archiving. PDF/A requires all fonts to be embedded, prohibits encryption, and disallows external content references.
Courts, archives, and regulatory bodies often mandate PDF/A format to ensure documents remain accessible and unaltered for decades.
Try our PDF/A Converter tool →Permission Password
Another name for the owner password. It restricts specific actions like printing, editing, or copying content from a PDF while still allowing the document to be opened and viewed.
Authors use permission passwords to share read-only versions of their work, preventing unauthorized editing or text extraction.
Try our Protect PDF tool →R
Rasterization
The process of converting vector graphics and text in a PDF into a pixel-based (bitmap) image. Each page becomes a flat image at a specified resolution.
Converting a PDF to JPG images involves rasterizing each page, which is useful for sharing previews or embedding in presentations.
Try our PDF to JPG tool →RC4 Encryption
A stream cipher historically used in PDF encryption (40-bit and 128-bit variants). RC4 is now considered less secure than AES and is deprecated in modern PDF specifications.
Older PDF files may use RC4-128 encryption, which some modern tools can remove to allow upgrading the document to stronger AES encryption.
Try our Unlock PDF tool →Redaction
The permanent and irreversible removal of sensitive content (text, images, or metadata) from a PDF. Unlike blacking out text with a marker, true redaction deletes the underlying data.
Legal teams redact Social Security numbers, financial figures, and privileged information from court filings before public release.
Try our Edit PDF tool →Resolution
The level of detail in a rasterized image, typically measured in DPI (dots per inch) or PPI (pixels per inch). Higher resolution means sharper output but larger file sizes.
Exporting a PDF page at 300 DPI produces print-quality images, while 72 DPI is sufficient for screen display and keeps file sizes small.
S
Split
The process of dividing a single PDF into multiple separate files, either by page range, individual pages, or at specific breakpoints.
Splitting a 200-page manual into individual chapters makes it easier to distribute, email, or upload sections that have strict file size limits.
Try our Split PDF tool →Stamp
A visual mark applied to PDF pages, such as "CONFIDENTIAL," "DRAFT," "APPROVED," or a custom image. Stamps are similar to watermarks but are typically more prominent and often placed on specific pages.
Adding a "DRAFT" stamp to each page of a proposal makes it clear the document is not yet finalized and should not be distributed.
Try our Watermark PDF tool →Stream Object
A PDF internal object that contains a sequence of bytes, typically used to store page content, embedded images, fonts, and other binary data. Streams can be compressed using various filters.
When a PDF compressor optimizes stream objects, it re-encodes the binary data with more efficient algorithms to reduce the overall file size.
T
Tagged PDF
A PDF that contains a logical structure tree defining the reading order, headings, paragraphs, lists, tables, and alternative text for images. Tagged PDFs are essential for accessibility.
Screen readers rely on PDF tags to correctly read documents aloud to visually impaired users, making tagging a legal requirement for many government documents.
Transparency
A PDF feature that allows objects to be partially see-through, enabling effects like semi-transparent watermarks, drop shadows, and overlapping elements with blending modes.
Transparent watermarks are placed over PDF content so the underlying text and images remain visible while the watermark is still clearly legible.
Try our Watermark PDF tool →Trim Box
A PDF page boundary that defines the intended final dimensions of the page after trimming in a print production workflow. It is typically smaller than the media box and may include bleed marks outside it.
Print shops use the trim box to know exactly where to cut the paper, ensuring the finished product has clean edges without white borders.
U
User Password
A PDF password required to open and view the document. Without this password, the file cannot be accessed at all, unlike the owner password which only restricts certain actions.
Setting a user password on a confidential report ensures that only people who know the password can open and read the document.
Try our Protect PDF tool →UTF-8
A character encoding standard capable of representing every character in the Unicode standard. Modern PDFs use UTF-8 for metadata and bookmarks to support international characters.
UTF-8 encoding ensures that PDF bookmarks and metadata display correctly in any language, from English to Chinese to Arabic.
V
Vector Graphics
Graphics defined by mathematical equations (points, lines, curves, and shapes) rather than pixels. Vector elements in PDFs remain sharp at any zoom level or print resolution.
Logos and diagrams stored as vector graphics in a PDF look crisp whether you view them on a phone screen or print them on a billboard.
Version (PDF)
The specification version of a PDF file, ranging from PDF 1.0 (1993) to PDF 2.0 (2017). Each version introduces new features such as encryption methods, transparency, and tagging.
A PDF created with version 2.0 features may not render correctly in older viewers, so saving to version 1.7 maximizes compatibility.
W
Watermark
Text or an image overlaid on PDF pages, typically semi-transparent, used to indicate status (draft, confidential), ownership, or branding without obscuring the underlying content.
Adding a company logo as a watermark to every page of a proposal reinforces branding while keeping all content fully readable.
Try our Watermark PDF tool →Web Worker
A browser API that runs JavaScript in a background thread, separate from the main page. Web Workers prevent heavy computations from freezing the user interface.
PDF Smaller uses Web Workers to compress large files in the background, keeping the interface responsive so you can continue interacting with the page.
WebAssembly (WASM)
A binary instruction format that allows code written in languages like C and Rust to run in the browser at near-native speed. It enables computationally intensive tasks that pure JavaScript cannot perform efficiently.
PDF Smaller loads the MozJPEG encoder as a WebAssembly module, achieving compression speeds comparable to desktop software while running entirely in the browser.
Try our Compress PDF tool →X
XMP Metadata
Extensible Metadata Platform data embedded in a PDF using XML format. XMP stores detailed information including authorship, copyright, description, and custom properties in a standardized way.
Photographers and publishers embed XMP metadata in PDFs to preserve copyright information and licensing terms alongside the document content.
XRef Table
Short for Cross-Reference Table. An index within a PDF file that records the byte position of every object, enabling PDF readers to jump directly to any page without scanning the entire file.
When a PDF fails to open with an error about an invalid XRef table, it usually means the file was corrupted during download or transfer.
Try our Repair PDF tool →Put These Concepts to Work
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