Definition
Matterhorn Protocol
The industry checklist enumerating PDF/UA conformance as concrete checkpoints — divided into machine-verifiable checks (what automated tools test) and human-judgment items (like whether reading order makes sense).
Published by the PDF Association, Matterhorn is why different checkers agree: they implement the same checkpoint list. The machine/human split is also the honest boundary of automation — tools verify structure exists and is well-formed; humans judge whether it's right.
Related tools & guides
Check whether your PDF is accessible
Ceereo audits any born-digital PDF against PDF/UA and WCAG machine checks for free, then writes the missing accessibility layer into the same file. The document looks exactly the same afterwards.