PDFlo is an independent field guide to the Portable Document Format — where it came from, how it works inside, and the standards that keep documents readable for decades. No software to install, nothing to upload. Just documentation, written plainly.
Browse the guidesThe Portable Document Format describes a fixed-layout document independently of the software, hardware, or operating system used to view it. Text, fonts, vector graphics, and images travel together in one self-contained file that prints and displays identically everywhere.
Every PDF has four parts: a header declaring the version, a body of numbered objects (pages, fonts, images), a cross-reference table mapping byte offsets, and a trailer pointing to it all. Readers start at the end of the file and walk backwards.
Content streams are squeezed with named filters: FlateDecode (the same deflate algorithm as ZIP) for text and vectors, DCTDecode (JPEG) for photographs, and JBIG2 or CCITT for scanned black-and-white pages. A single file can mix all of them.
A PDF either references a font by name or embeds it outright — often as a subset containing only the glyphs the document uses. Embedding is what makes a file truly portable; missing fonts are why some documents render with the wrong typeface.
Digital signatures embed a cryptographic checksum over the file's bytes, so any later edit is detectable. Signing appends a new revision rather than rewriting the document, preserving everything that came before — ink signatures on paper, upgraded.
PDF/A (ISO 19005) is the archival profile: fonts must be embedded, encryption and external references are forbidden, and color is device-independent. Libraries and governments use it so files still open correctly in fifty years.
| Profile | Standard | Purpose |
|---|---|---|
| PDF/A | ISO 19005 | Long-term archiving — self-contained, no external dependencies |
| PDF/X | ISO 15930 | Print production — CMYK color, embedded fonts, press-ready output |
| PDF/E | ISO 24517 | Engineering documents — technical drawings and 3D content |
| PDF/UA | ISO 14289 | Universal accessibility — tagged structure for assistive technology |
| PAdES | ETSI | Advanced electronic signatures for legally binding documents |
The PDF format quietly carries contracts, medical records, boarding passes, and court filings — trillions of pages a year — yet its inner workings are documented mostly in dense specifications. PDFlo collects what is worth knowing about the format in one calm, readable place.
There is nothing to download and nothing to install here. It is a library, not a toolbox.