Technical details about AI-assisted checks

Background

When is a PDF document accessible? This question is not so easy to answer. The document must pass no fewer than 31 tests with a total of 137 conditions. These tests are listed in detail in the Matterhorn Protocol. It is the checklist for PDF accessibility.

Of the 136 conditions in the protocol, 41 require human judgment. For example, “classic” software cannot recognize whether a paragraph belongs to a title or to the body text (i.e., whether an H or P tag would be correct). All PDF testing tools have failed to solve this problem so far.

PAC 2026 introduces artificial intelligence (AI) features. With these new features, PAC can cover some of the Matterhorn Protocol's checkpoints that previously required human judgment. PAC 2026 thus significantly reduces the manual testing effort and makes it easier for beginners in particular to access professional PDF/UA testing.

Functionality

The AI built into PAC 2026 was trained using many accessible PDFs. It learned how to recognize semantically correct tagged areas based on these documents.

The AI runs exclusively locally, on the computer on which PAC is running. It does not access cloud services or the internet in any way. Accordingly, your PDF documents always remain on your computer and are not used to train the AI.

Semantic areas (structural elements)

The AI-supported checks recognize the following semantic areas of a document:

  • Body text (P)
  • Headings (H)
  • Lists (L)
  • Tables (Table)
  • Images and graphics (Figure)
  • Tables of contents (TOC)
  • Captions (Caption)
  • Footnotes and notes (Note)

Disabling AI-supported checks

Starting with PAC 2026.1, AI-supported checks can be disabled via a registry key. To do so, follow these steps:

  1. Launch the Registry Editor (regedit.exe).
  2. Navigate to HKEY_CURRENT_USER\Software\axes4\PAC and add a DWORD (32-bit) Value named EnableAIChecks.
  3. A value of 0 for this key means that AI-powered checks will not be performed. A value of 1 (or if the key is missing) means that the checks will be performed.

The DWORD value can be added in four different locations. These are read and processed in the following order of precedence:

  1. HKEY_LOCAL_MACHINE\SOFTWARE\Policies\axes4\PAC
  2. HKEY_CURRENT_USER\Software\Policies\axes4\PAC
  3. HKEY_LOCAL_MACHINE\SOFTWARE\axes4\PAC
  4. HKEY_CURRENT_USER\Software\axes4\PAC