PDF stands for Portable Document Format. It was introduced by Adobe in 1993 and became an open ISO standard (ISO 32000) in 2008. A PDF stores a full description of the page — text, vectors, raster images, embedded fonts — so anyone who opens the file sees the same layout as the author.
Because layout is fixed, PDFs are the default for invoices, contracts, tax forms, resumes, e-books, and government paperwork. Modern PDFs can also carry form fields, digital signatures, embedded files, JavaScript, and encryption for password protection.
PDFs are read with any modern browser, macOS Preview, Adobe Acrobat, or free viewers. Editing usually means running the file through a converter or a browser-based tool like the ones on this site — you rarely edit the raw bytes by hand.