PDF Glossary: Terms & Definitions

PDF technology comes with its own vocabulary. Whether you're a beginner or an experienced user, this glossary will help you understand the terms and concepts behind PDF tools and workflows.

A

Adobe Acrobat — The original PDF software created by Adobe Systems. Available in Reader (free, view-only), Standard, and Pro (paid, full editing) versions.

Annotation — Any markup added to a PDF, including highlights, comments, sticky notes, underlines, strikethroughs, and drawing markups. Annotations can be reviewed and exported.

C

Compression — Reducing the file size of a PDF by optimizing images, removing redundant data, and applying compression algorithms. Can be lossless (no quality loss) or lossy (some quality reduction).

Conversion — Transforming a PDF into another format (Word, Excel, JPG, etc.) or converting another format into PDF. May involve OCR for scanned documents.

D

Digital Signature — A cryptographic signature that verifies the authenticity and integrity of a PDF document. Unlike a handwritten signature image, digital signatures use encryption to prove the document hasn't been tampered with.

DPI (Dots Per Inch) — A measure of image resolution. Higher DPI means sharper images but larger file sizes. 300 DPI is standard for print; 72-150 DPI is sufficient for screen viewing.

E

Encryption — Scrambling the contents of a PDF so it can only be opened with a password or digital certificate. PDF supports 40-bit, 128-bit, and 256-bit AES encryption.

eSignature — An electronic representation of a signature, often a typed name, drawn signature, or uploaded image. Legally binding in many jurisdictions but not as secure as a digital signature.

F

Fillable Form — A PDF with interactive form fields (text boxes, checkboxes, radio buttons, dropdowns) that users can fill in electronically. Created with tools like Adobe Acrobat Pro or PDFelement.

Flattening — Merging annotations, form fields, and layers into the main document content. Flattened PDFs can no longer be edited annotation-by-annotation and have smaller file sizes.

J

JPEG/JPG — A common image format. PDFs can contain embedded JPEG images, and PDF pages can be exported as JPEG files. JPEG uses lossy compression.

L

Layers (OCG) — Optional Content Groups, allowing different elements of a PDF to be shown or hidden independently. Useful for multi-language documents, design variations, or complex diagrams.

Lossless Compression — Compression that reduces file size without any loss of quality. The decompressed file is identical to the original. Common in PDFs using Flate or LZW compression.

Lossy Compression — Compression that reduces file size by discarding some data, resulting in minor quality loss. Typically applied to images within PDFs (JPEG, JPEG2000).

M

Merge — Combining two or more PDF files into a single document. Pages from each source file are concatenated in the order specified.

Metadata — Hidden information embedded in a PDF, including title, author, subject, keywords, creation date, modification date, and software used. Can be viewed and edited in most PDF tools.

O

OCR (Optical Character Recognition) — Technology that recognizes text in scanned images or image-based PDFs and converts it into selectable, editable text. Essential for editing scanned documents. Accuracy depends on image quality and language support.

Open Standard — PDF was published as an open standard (ISO 32000) in 2008, meaning it's no longer controlled solely by Adobe. Any software developer can create PDF tools without licensing fees.

P

PDF/A — A specialized PDF format for long-term archiving. PDF/A files embed all fonts, avoid external dependencies, and restrict features that could cause compatibility issues in the future.

PDF/UA — Universal Accessibility PDF format, designed to ensure PDFs are accessible to users with disabilities, including screen reader compatibility and proper document structure.

Portability — The "P" in PDF. PDFs can be opened and viewed on virtually any device and operating system without the original software that created them, with consistent appearance.

R

Resolution — The detail level of images in a PDF, measured in DPI. Higher resolution means sharper images but larger files. Web PDFs typically use 72-150 DPI; print PDFs use 300+ DPI.

S

Split — Dividing a single PDF into multiple smaller documents. Can be done by page ranges, extracting specific pages, or splitting every N pages.

Watermark — Text or an image overlaid on PDF pages, often used to indicate document status ("Draft", "Confidential") or branding. Can be opaque or semi-transparent.

T

Tagged PDF — A PDF with hidden structural tags (headings, paragraphs, lists, tables) that improve accessibility, reflow on mobile devices, and conversion to other formats. Required for PDF/UA compliance.

W

Watermark — See above. A text or image overlay applied to PDF pages for branding, status indication, or copyright protection.

Whiteout — A PDF editing technique where a white rectangle is placed over content to "erase" it. The underlying content may still be recoverable, so whiteout is not secure for removing sensitive information (use redaction instead).

X

XFA (XML Forms Architecture) — An older XML-based form format used in some PDFs, particularly from Adobe LiveCycle. XFA forms have limited compatibility with non-Adobe PDF readers and are being phased out in favor of standard AcroForms.