The PDF Format, Explained Simply

A PDF is a self-contained document: text, fonts (or subsets), vector drawings and images, plus a cross-reference table so readers can jump to any page instantly. That self-containment is why a PDF prints identically on any machine.

Text in a PDF is positioned glyph by glyph with coordinates, unlike HTML which flows. This precision is what makes PDFs print-perfect — and what makes reflowing them on small screens awkward.

Fonts are the classic gotcha: a PDF that references but does not embed a font renders with a substitute on other machines. Embedding (or subsetting) fonts fixes it at the cost of file size.

Real PDFs come in flavors: born-digital PDFs with selectable text, scanned image-only PDFs that need OCR to become searchable, and tagged accessible PDFs with reading order for screen readers.

You can see the format in action with the RovoTools PDF Creator tool, which writes a real downloadable PDF directly in your browser — a good mental model of the format with no server involved.