Matterhorn Protocol

The Matterhorn Protocol

The PDF/UA-1 standard defines rules to which a document must obey in order to conform to the standard. The PDF/UA Competence Center, an initiative of the PDF Association, compiled a comprehensive list of all possible PDF/UA-1 standard violations. This list, called the Matterhorn Protocol, provides guidance to software developers and document testers for achieving full accessibility.

The Matterhorn Protocol rephrases the standard conformance requirements and lists all possible violations as 136 »failure conditions« which are grouped in 31 checkpoints. While most failure conditions can be identified by software, others require human judgment. Some examples for failure conditions which can be identified by software:

  • An image is missing alternative or replacement text.
  • A link annotation is not nested within a Link tag.
  • XMP metadata does not include a document title.

Some examples for failure conditions which require human judgment:

  • Heading text is not tagged with a heading tag.
  • Irrelevant content is not tagged as Artifact.
  • OCR’ed text contains character identification errors.
  • Content is presented as a list, but is not tagged as a list.

The Matterhorn Protocol can be used as a guideline for identifying PDF/UA-1 standard violations. For example, the PDF Accessibility Checker (PAC) is a PDF/UA validation tool which is freely available from the Swiss non-profit organization Access for All. PAC implements the Matterhorn Protocol. Also, the veraPDF open-source PDF validator supports PDF/UA-1 validation according to the Matterhorn Protocol.