Reference

PDF glossary

Short, plain-language definitions of the terms you meet when you merge, convert and share PDF and Office documents — 31 terms in all.

Updated October 21, 2026

PDF basics

PDF: PDF (Portable Document Format) is a file format that shows a document the same way on every device, with its text, fonts, images and layout fixed in place. Adobe introduced it in 1993, and it has been an open ISO standard (ISO 32000) since 2008.
PDF/A: PDF/A is a version of PDF standardised for long-term archiving (ISO 19005). It requires every font to be embedded and forbids features that could stop a file opening in the future, such as encryption and links to external content.
Merging PDFs: Merging PDFs means combining two or more PDF files into one document, with the pages of each file following one another in the order you choose. You can do it free with Merge PDF.
Splitting a PDF: Splitting a PDF means dividing one document into several smaller files — for example one file per page, per chapter or per range of pages.
Watermark: A watermark is text or an image — such as a logo, “Confidential” or “Draft” — placed over or behind the content of each page. Some free online tools stamp their own watermark on files they process; Mergevanta never does (see how to merge without one).
Text layer: A PDF's text layer holds the actual characters on the page, which is what lets you select, copy and search the text. Scanned PDFs have no text layer unless OCR has added one.
Scanned PDF: A scanned PDF contains pictures of pages rather than real text, so its words can't be searched or copied until it has been through OCR. See how to combine scanned pages into one PDF.
OCR: OCR (optical character recognition) turns pictures of text — scanned pages, photos of documents, screenshots — into real text that can be searched, selected and edited.
Bookmarks (outline): Bookmarks, also called the outline, are a clickable table of contents in a PDF reader's sidebar that jumps to headings or pages in the document.
PDF metadata: PDF metadata is information stored inside a file about the file itself — such as its title, author, creation date and the software that produced it. It appears in a PDF reader's document properties.
Flattening: Flattening a PDF merges form fields, annotations and layers into the page content, so they can no longer be edited and look the same in every reader.
Font embedding: Font embedding stores the fonts a document uses inside the PDF itself, so the text looks the same on devices that don't have those fonts installed.

Document file formats

DOCX: DOCX is Microsoft Word's default file format since Word 2007: a ZIP package of XML files (Office Open XML) holding a document's text, styles, images and settings. Combine several with Merge Word or turn one into a PDF with Word to PDF.
DOC: DOC is the binary file format used by Microsoft Word 97–2003. Modern Word still opens it, but DOCX has replaced it as the default.
PPTX: PPTX is PowerPoint's default file format since PowerPoint 2007: an Office Open XML package containing slides, layouts, themes, speaker notes and media. Combine decks with Merge PowerPoint.
XLSX: XLSX is Excel's default workbook format since Excel 2007: an Office Open XML package that stores worksheets, formulas, formatting and charts. Combine workbooks with Merge Excel.
CSV: CSV (comma-separated values) is a plain-text format for tables, with one row per line and commas between the columns. It holds data only — no formatting, formulas or multiple sheets.
Office Open XML: Office Open XML (OOXML) is the ZIP-and-XML file format family behind DOCX, PPTX and XLSX, standardised as ECMA-376 and ISO/IEC 29500.

Images and quality

JPEG (JPG): JPEG, also written JPG, is an image format that uses lossy compression to keep photos small by discarding detail the eye is unlikely to notice. Each re-save can lose a little more quality, which is why JPG to PDF embeds JPEGs without re-compressing them.
PNG: PNG is an image format with lossless compression and support for transparency, which makes it ideal for screenshots, logos and diagrams. Turn PNGs into a document with PNG to PDF.
Lossless compression: Lossless compression makes a file smaller without discarding any information, so the original can be restored exactly. PNG images and ZIP archives use lossless compression.
Lossy compression: Lossy compression makes files much smaller by permanently discarding some detail. JPEG photos and most streaming video use lossy compression.
Vector graphics: Vector graphics describe shapes and text mathematically, so they stay perfectly sharp at any zoom level. Most text and line art in a PDF is vector.
Raster image: A raster image is a grid of pixels, such as a photo or a screenshot. It looks blurry or blocky when enlarged beyond its resolution.
DPI: DPI (dots per inch) measures how much detail a printed or scanned image has. 300 DPI is the usual standard for sharp printed documents and scans.

Pages and printing

A4: A4 is the international paper size of 210 × 297 mm (8.27 × 11.69 in), defined by ISO 216 and used for documents in most of the world.
US Letter: US Letter is the standard paper size in the United States and Canada, measuring 8.5 × 11 inches (215.9 × 279.4 mm).
Portrait and landscape: Portrait pages are taller than they are wide; landscape pages are wider than they are tall. A merged PDF keeps each page's own orientation.

Security and privacy

Password-protected PDF: A password-protected PDF is encrypted. An open (user) password is needed to view it at all, while a permissions (owner) password only restricts actions such as printing, copying or editing.
End-to-end encryption: End-to-end encryption scrambles data on your device so that only the intended recipient can read it; anyone in between, including network providers, sees only ciphertext. Read more in Is it safe to merge PDFs online?
Post-quantum cryptography: Post-quantum cryptography uses algorithms designed to resist attacks by future quantum computers, such as ML-KEM, which NIST standardised as FIPS 203 in 2024.

Want step-by-step help instead? Browse the PDF guides.