Paper, but virtual: documents that look the same everywhere.
Where it came from
PDF grew out of Adobe's Camelot project (1991) by John Warnock: a universal format that would preserve document layout on any machine — 'paper transported over wires'. Version 1.0 shipped in 1993; Adobe kept it proprietary until 2008, when it was released to ISO as an open standard.
The gamble paid off beyond expectations: today PDF is how contracts, articles, books and receipts travel.
How it works
A PDF is not a stream of text — it's a page *description* language. Pages contain positioned glyphs (often with custom embedded fonts and encodings), vector graphics and raster images. Text is drawn, not written: 'place glyph #42 at (100, 200)'.
That's why extracting text is genuinely hard: the extractor must reverse glyph mappings back into Unicode, deal with ligatures, hyphenation and reading order. UWC uses pdfjs-dist (the engine behind Firefox's viewer) to walk pages and reconstruct readable text.