Every PDF term —
explained simply
84 terms from AES encryption to ZUGFeRD — clear definitions, practical examples, and links to relevant tools. No jargon, no fluff.
A
AcroForm
A type of interactive form field in PDFs — text boxes, checkboxes, radio buttons, dropdowns. AcroForms are the standard format for fillable PDF forms and are supported by all PDF readers.
AES Encryption
Advanced Encryption Standard — the algorithm used to password-protect PDFs. PDFly uses AES-256, the same standard banks and governments use. Without the password, the file is unreadable.
Protect PDF tool →Annotation
Any mark added to a PDF — highlights, sticky notes, stamps, drawings, or comments. Annotations are layered on top of the page content without altering the original document.
Artifact
A PDF content element that is decorative rather than meaningful — page numbers, running headers, background images. Artifacts are tagged so screen readers skip them, improving accessibility.
ASCII
American Standard Code for Information Interchange — a character encoding standard for basic text. PDF text is typically encoded in Unicode, but ASCII is used for some metadata and internal structures.
B
Batch Processing
Applying the same operation (compress, convert, watermark) to multiple PDFs at once. PDFly supports batch processing to save hours of repetitive work.
Batch Process tool →Bleed
The area of a PDF page that extends beyond the trim edge. Bleed ensures no white borders appear after printing and trimming. Standard bleed is 3mm (0.125in) on all sides.
Bookmark
A navigational link in a PDF's sidebar panel that jumps to a specific page or section. Bookmarks are created from headings and provide a table-of-contents-like experience.
Booklet
A PDF printing layout where pages are arranged so that when printed double-sided and folded, they appear in correct reading order. Common for brochures and small booklets.
C
CBR/CBZ
Comic Book Archive formats — essentially renamed RAR/ZIP files containing image sequences. Often converted to/from PDF for easier reading on universal viewers.
CMYK
Cyan, Magenta, Yellow, Key (black) — the color model used for print. PDFs intended for professional printing should use CMYK color space, while screen viewing uses RGB.
Compression
Reducing a PDF's file size by removing redundant data, downsampling images, and optimizing fonts. PDFly offers three levels: Light (~30% reduction), Medium (~60%), and Extreme (~80%).
Compress PDF tool →Cross-Reference
A link within a PDF that points to another section, figure, or page. Cross-references update automatically when content is reordered, keeping navigation accurate.
Crop Marks
Thin lines printed at the corners of a PDF page to indicate where the paper should be trimmed. Essential for professional printing to achieve precise final dimensions.
D
Digital Signature
A cryptographic stamp that verifies a PDF's authenticity and integrity. Unlike a scanned image of your signature, a digital signature proves the document hasn't been altered after signing.
Document Properties
Metadata embedded in a PDF — title, author, subject, keywords, creation date, modification date, and the software used to create the file. Can be viewed and edited with PDFly's metadata tools.
Metadata Remover tool →Downsampling
Reducing the resolution of images embedded in a PDF to decrease file size. Downsampling to 150 DPI for screen viewing has no visible quality loss but can halve file size.
DPI (Dots Per Inch)
A measure of image resolution. 72 DPI is standard for web/screen viewing, 150 DPI for basic printing, and 300 DPI for professional print quality. Higher DPI = sharper images but larger file sizes.
E
Embedded File
Any file attached inside a PDF — other PDFs, spreadsheets, images, or documents. PDF/A-3 uses embedded files to bundle machine-readable data alongside the human-readable PDF.
Embedded Font
A font that's included inside the PDF file itself, ensuring the document looks the same on every device. Without embedded fonts, PDFs may render differently depending on what fonts the viewer has installed.
Encryption
Scrambling PDF content so only someone with the correct password can read it. PDF encryption uses either RC4 (older, less secure) or AES (modern, recommended) algorithms.
Protect PDF tool →Extract
Pulling specific pages, text, or images out of a PDF as separate files. For example, extracting pages 5-10 as a new PDF, or extracting all text as a .txt file.
Split PDF tool →F
Flatten
Converting interactive PDF elements (form fields, annotations, layers) into static content that can't be edited. Flattening is essential before sharing filled forms to prevent further modification.
Font Subsetting
Including only the characters actually used in a PDF rather than the entire font file. Subsetting can reduce font size by up to 90%, significantly lowering overall file size.
Form Field
An interactive area in a PDF where users can enter data — text fields, checkboxes, radio buttons, dropdown lists, or signature fields. Common in applications, contracts, and surveys.
G
Ghostscript
An open-source interpreter for PostScript and PDF files. Many PDF tools use Ghostscript under the hood for rendering, converting, and compressing PDFs.
Glyph
The visual representation of a single character in a font. A font file contains glyphs for each character, and PDFs reference glyph IDs to render text efficiently.
Grayscale
A color mode using only shades of gray (black to white). Converting a PDF to grayscale reduces file size and is required for some legal and archival documents.
H
Hex Color
A six-digit hexadecimal code representing a color (e.g., #2563EB for blue). PDFs can use hex colors in annotations, form fields, and vector graphics for precise color matching.
HIPAA
Health Insurance Portability and Accountability Act — a US law requiring medical documents to be protected. PDFly's client-side processing makes it HIPAA-compliant since files never leave the user's device.
Hyperlink
A clickable link inside a PDF that navigates to another page, a website, or an email address. Hyperlinks can be text-based or attached to images and buttons.
I
Inch
A unit of measurement used in PDF page sizes and margins. US Letter (8.5 × 11 inches) and Legal (8.5 × 14 inches) are common inch-based page sizes.
Incremental Update
A PDF feature where edits are appended to the end of the file rather than rewriting the entire document. This preserves previous versions and speeds up saving, but increases file size over time.
Interactive PDF
A PDF with interactive elements — forms, buttons, multimedia, hyperlinks, and bookmarks. Interactive PDFs are commonly used for presentations, applications, and digital workbooks.
J
JavaScript in PDF
A scripting language embedded in interactive PDFs to add logic — form validation, calculated fields, dynamic show/hide elements. Not all PDF viewers support JavaScript equally.
JPEG (JPG)
A common image format using lossy compression. PDFs can contain JPEG images, and PDF pages can be exported as JPEG files. Best for photographs and images with many colors.
PDF to JPG tool →K
Kerning
The spacing between individual letters in text. Proper kerning improves readability and visual appeal. PDF preserves kerning information from the original document's embedded fonts.
Keywords
Descriptive terms stored in a PDF's metadata to improve searchability. Keywords help document management systems categorize and retrieve files. Can be edited with PDFly's metadata tools.
L
Layers (OCG)
Optional Content Groups — layers in a PDF that can be shown or hidden independently. Used for multi-language documents, architectural plans, and interactive maps.
Linearized PDF
Also called "Fast Web View" — a PDF optimized for web viewing where the first page loads before the entire file downloads. Essential for large PDFs served online.
Lossless Compression
Compression that reduces file size without any quality loss. Text and vector graphics in PDFs use lossless compression. Image compression is typically lossy (some quality loss).
Lossy Compression
Compression that reduces file size by discarding some data, resulting in minor quality loss. JPEG images in PDFs use lossy compression. The trade-off is smaller files at the cost of slight image degradation.
M
Merge
Combining multiple PDF files into a single document. PDFly's merge tool preserves page order and lets you drag files into the desired sequence before combining.
Merge PDF tool →Metadata
Hidden information embedded in a PDF — author name, creation date, edit history, GPS coordinates from scans, software used. Metadata can reveal sensitive info, so it should be stripped before sharing externally.
Metadata Remover tool →MIME Type
Multipurpose Internet Mail Extensions — the standard that defines `application/pdf` as the MIME type for PDF files. Browsers and servers use this to correctly handle PDF content.
N
Named Destination
A bookmarked location within a PDF that can be linked to directly via URL. Useful for long documents where you want to share a specific section rather than the first page.
Nonlinear PDF
A standard PDF where the entire file must download before any page can be displayed. Unlike linearized PDFs, nonlinear files are slower to open over the web, especially for large documents.
O
OCR (Optical Character Recognition)
Technology that converts scanned images of text into searchable, selectable digital text. PDFly's OCR supports 100+ languages and preserves the original document layout.
OCR PDF tool →Outline
The hierarchical tree of bookmarks in a PDF, also called the document outline. Each entry links to a destination in the document, providing quick navigation for long files.
Owner Password
A password that controls PDF permissions — printing, copying, editing, commenting. Different from the user password (which controls who can open the file). PDFly lets you set both independently.
P
PDF/A
An ISO-standardized version of PDF designed for long-term archival. PDF/A requires all fonts to be embedded, prohibits external references, and ensures the document will render identically decades from now.
PDF/E
A PDF standard for engineering documents. PDF/E supports 3D models, embedded CAD data, and large-format drawings, making it ideal for technical and engineering workflows.
PDF/UA
PDF/Universal Accessibility — an ISO standard for accessible PDFs. Requires tagged content, alt text, logical reading order, and proper heading structure for assistive technologies.
PDF/X
A PDF standard for graphic exchange in professional printing. PDF/X ensures color profiles, bleed, and trapping are correctly embedded for print production.
Pixelation
When an image appears blocky or blurry due to low resolution or excessive compression. Avoided by using appropriate DPI settings and moderate compression levels.
Q
QR Code in PDF
A 2D barcode that can be embedded in a PDF to link to websites, download apps, or verify document authenticity. Commonly used on tickets, menus, and marketing materials.
Quartz
Apple's 2D graphics rendering engine used by macOS and iOS to display PDFs. Quartz provides native PDF support in Preview and other Apple applications.
R
Raster Image
An image made of pixels, like a photograph. Unlike vector graphics, raster images lose quality when scaled up. PDFs can contain both raster and vector content.
RC4 Encryption
An older encryption algorithm used in early PDF versions (1.1–1.5). RC4 is considered insecure by modern standards and has been replaced by AES in newer PDF specifications.
Redaction
Permanently removing sensitive content from a PDF — not just covering it with a black box, but stripping the underlying text/data so it can't be recovered. Critical for legal and government documents.
Redact PDF tool →Resolution
The clarity of images in a PDF, measured in DPI. Screen resolution is typically 72-96 DPI, while print requires 300 DPI or higher. Higher resolution means larger file size.
RGB
Red, Green, Blue — the color model used for screens. PDFs viewed on devices use RGB, but files intended for print should be converted to CMYK for accurate color reproduction.
S
Split
Dividing a single PDF into multiple files — by page ranges, every N pages, or by bookmarks. Useful for extracting chapters from reports or separating multi-document scans.
Split PDF tool →Sticky Note
A type of PDF annotation that appears as a small pop-up icon on the page. Sticky notes can contain comments, review feedback, or instructions without modifying the page content.
Subset Font
A font that includes only the characters used in the document rather than the full character set. Subsetting dramatically reduces file size, especially for CJK fonts with thousands of glyphs.
SVG
Scalable Vector Graphics — an image format that scales without quality loss. SVGs can be embedded in PDFs for crisp logos and diagrams at any zoom level.
T
Tagged PDF
A PDF with structured tags that define reading order, headings, tables, and alt text for images. Tagged PDFs are essential for accessibility — screen readers use tags to navigate documents.
Text Extraction
Pulling all text content out of a PDF as plain text or a Word document. For scanned PDFs, OCR must be run first to make the text extractable.
PDF to Word tool →Thumbnail
A small preview image of each PDF page shown in the sidebar of PDF viewers. Thumbnails help users navigate multi-page documents visually. PDFly generates thumbnails for page management.
TIFF
Tagged Image File Format — a high-quality image format often used for scanned documents. TIFF supports multiple pages and lossless compression, making it a common source for PDF conversion.
U
Unicode
A text encoding standard that supports virtually all languages and characters. Modern PDFs use Unicode, ensuring text displays correctly regardless of the viewer's language settings.
User Password
The password required to open and view a PDF. Different from the owner password, which controls permissions. A PDF can have a user password, an owner password, both, or neither.
V
Vector Graphics
Images defined by mathematical paths rather than pixels. Vector graphics in PDFs (logos, diagrams, charts) scale infinitely without quality loss, unlike raster images.
Viewer Preferences
Settings stored in a PDF that control how it opens — page layout, zoom level, whether the toolbar is hidden, and which navigation panel is displayed. Authors use these to create a guided reading experience.
W
Watermark
A text or image overlay on PDF pages — "DRAFT", "CONFIDENTIAL", a company logo. Watermarks identify document status or ownership without obscuring content when set to low opacity.
Watermark PDF tool →WebAssembly (WASM)
A binary instruction format that runs code at near-native speed in web browsers. PDFly uses WebAssembly to process PDFs entirely client-side — no server uploads needed.
Web Capture
A PDF feature that converts web pages into PDF content, preserving links and layout. Useful for archiving web content or creating offline-readable documentation.
X
XFA (XML Forms Architecture)
An Adobe-specific format for dynamic PDF forms. XFA forms can have reflowing content and complex logic but aren't supported by all PDF readers. Being phased out in favor of AcroForms.
XObject
A reusable object in a PDF — an image, form, or pattern that can be referenced multiple times without duplicating data. XObjects keep PDFs efficient by avoiding redundant content.
XMP Metadata
Extensible Metadata Platform — Adobe's XML-based standard for embedding metadata in PDFs. XMP can include custom fields like copyright info, workflow history, and rights management.
Y
YCbCr
A color space used in JPEG compression within PDFs. Separates brightness (Y) from color (Cb, Cr) information, allowing more aggressive compression of color data with minimal visual impact.
Z
Z-Order
The stacking order of overlapping elements in a PDF. Z-order determines which objects appear in front of others — critical for layered content, watermarks, and annotations.
ZUGFeRD
A German standard for electronic invoices using PDF/A-3 with embedded XML data. Combines human-readable PDF with machine-readable invoice data for automated processing.
Zoom
The magnification level at which a PDF is viewed. PDF viewers typically support 10% to 6400% zoom. Vector content remains crisp at any zoom level; raster images may pixelate when zoomed in.
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