A quick definition
A PDF file packages text, fonts, images, vector graphics, and layout instructions into a single self-contained document. When you open it, the file tells the viewer exactly where each element should appear on the page.
Because the formatting travels with the file, a PDF you create on a Mac looks identical when opened on Windows, Android, iOS, or a web browser.
How a PDF stores your document
Internally a PDF is a structured tree of objects: pages, fonts, images, and content streams. Each page references a content stream that lists every drawing instruction in order.
Fonts can be embedded directly in the file, which is why a PDF can guarantee identical typography even if the reader does not have those fonts installed.
Common uses for PDFs
PDFs are used anywhere fidelity matters: contracts, invoices, tax forms, academic papers, e-books, product manuals, and printable templates.
They are also a popular archival format because the ISO-standardized PDF/A variant is designed to remain readable for decades.
Why PDFs became the default
Universal compatibility, predictable printing, support for digital signatures, password protection, and the ability to combine many different content types in one file made PDFs the de facto standard for sharing finished documents.