What is Horn?

Horn is an open-source PDF/UA accessibility checker based on the Matterhorn Protocol 1.1. It validates PDF files against PDF/UA-1 (ISO 14289-1) and PDF/UA-2 (ISO 14289-2), targeting the machine-checkable failure conditions defined in the protocol.

Horn is designed as a cross-platform, CI/CD-ready alternative to PAC 2024.

Why Horn?

Most PDF accessibility checkers are slow, GUI-only, or platform-locked. Horn takes a different approach:

• Built in Rust for raw speed — validate 500+ PDFs per second with parallel processing
• Single binary with zero runtime dependencies — no JVM, no GUI frameworks
• CI/CD first — SARIF, JUnit XML, and JSON outputs integrate directly into your pipeline
• Cross-platform — Linux, macOS, and Windows from the same codebase

How it works

Horn uses a dual-parser architecture for optimal performance:

1. pdf_oxide parses the PDF structure tree eagerly (~4ms per file) and runs built-in PDF/UA-1 compliance checks
2. lopdf provides raw PDF object access and is initialized lazily — only when checks need low-level access

This means simple PDFs are validated extremely quickly, while complex documents still get thorough checking.

Matterhorn Protocol

The Matterhorn Protocol defines 136 failure conditions across 31 checkpoints for PDF/UA-1. Horn covers every condition in the protocol's tables (137 index entries in version 1.1, which added 13-008): the 87 machine-checkable ones are automated, the 48 human-judgment ones are reported as manual-review items and the 2 without a defined test are reported as not applicable, with every finding carrying the official Matterhorn index. Its 29 check modules cover, among others:

• Document metadata and language
• Structure tree and tagging
• Font embedding and encoding
• Heading hierarchy
• Table structure
• Image alt text
• Annotation accessibility
• List nesting
• And more

For PDF/UA-2 documents (pdfuaid:part 2) Horn adds 37 rules for the requirements that only exist in ISO 14289-2 — PDF 2.0 namespaces, structure destinations, annotation artifacts, pdfuaid:rev — using interim ua2:<clause>-<test> ids until the Matterhorn Protocol 2.0 is published. See the checks reference.

Run horn list-checks to see all registered checks.

Platforms

Horn is available as:

• CLI — the primary interface, suitable for local use and CI/CD
• GitHub Action — drop-in action with automatic SARIF upload
• Web app — browser-based validation via WebAssembly
• Desktop app — native app built with Tauri

License

Horn is licensed under the MIT license.