Definition
PDF is a fixed-layout document format introduced by Adobe in 1993 and now an ISO standard (ISO 32000).
How it works
A PDF packs text, fonts, vector and bitmap images, and layout descriptions into one file, so its rendering does not depend on the device, system or software viewing it—a print-like “what you see is what you get” model.
Usage
Contracts, papers, manuals and print-ready drafts—any document scenario that needs fixed layout and cross-platform exchange—is almost always shared as a PDF.
Related and confusable terms
PDF makes a different trade-off from Markdown: Markdown is a lightweight plain-text markup that favors content over layout and is easy to edit and version; PDF fixes the layout, favoring presentation over editing.