Horn

Open-source PDF/UA accessibility checker based on the Matterhorn Protocol.

Horn validates PDF files against PDF/UA-1 (ISO 14289-1), targeting the machine-checkable failure conditions defined in the Matterhorn Protocol 1.1. It is designed as a cross-platform, CI/CD-ready alternative to PAC 2024.

Features

• Fast: ~660 PDFs/second with parallel processing (release mode)
• CI/CD native: SARIF (GitHub Code Scanning), JUnit XML, JSON output formats
• Cross-platform: Linux, macOS, Windows — no JVM or GUI required
• Comprehensive: 9 check modules covering metadata, structure, fonts, headings, tables, images, annotations, and lists
• Extensible: Check trait for adding custom checks

Installation

Pre-built binaries

Download from GitHub Releases.

From source

cargo install --git https://github.com/focusring/horn.git

Docker

docker build -t horn .
docker run -v $(pwd):/data horn validate /data/document.pdf

Usage

# Single file
horn validate document.pdf

# Multiple files
horn validate *.pdf --format json

# Directory (recursive)
horn validate ./docs/ --recurse --format sarif -o results.sarif

# CI mode: fail pipeline if any errors
horn validate ./output/ --recurse --format junit --fail-on error -o results.xml

# Only fail on warnings or worse (ignore info-level findings)
horn validate doc.pdf --fail-on warning

# List available checks
horn list-checks

# Generate shell completions
horn completions bash > ~/.local/share/bash-completion/completions/horn
horn completions zsh > ~/.zfunc/_horn
horn completions fish > ~/.config/fish/completions/horn.fish

Exit codes

Code	Meaning
0	All files compliant
1	One or more files have findings at or above --fail-on severity
2	CLI or runtime error

GitHub Action

- uses: focusring/horn@v1
  with:
    path: ./output/
    format: sarif
    fail-on: error

The action automatically uploads SARIF results to GitHub Code Scanning when format: sarif.

CI/CD Examples

GitHub Actions (manual)

steps:
  - uses: actions/checkout@v4
  - name: Install Horn
    run: |
      curl -sL https://github.com/focusring/horn/releases/latest/download/horn-linux-x86_64 \
        -o /usr/local/bin/horn && chmod +x /usr/local/bin/horn
  - name: Validate PDFs
    run: horn validate ./docs/ --recurse --format sarif -o results.sarif
  - uses: github/codeql-action/upload-sarif@v3
    if: always()
    with:
      sarif_file: results.sarif

GitLab CI

pdf-accessibility:
  image: rust:latest
  script:
    - cargo install --git https://github.com/focusring/horn.git
    - horn validate ./docs/ --recurse --format junit -o results.xml
  artifacts:
    reports:
      junit: results.xml

Checks

Horn implements checks across these Matterhorn Protocol checkpoints:

Module	Checkpoint	What it validates
baseline	01-31	pdf_oxide built-in PDF/UA-1 validation
metadata	06	Document language, title display, PDF/UA identifier, XMP
structure	01/02/09	Tagged PDF, StructTreeRoot, MarkInfo, role mapping
fonts	31	Font embedding, ToUnicode CMaps, encoding
headings	14	H1-H6 hierarchy, no skipped levels
tables	15	TR/TH/TD structure, header identification
images	13	Figure alt text presence
annotations	28	Tab order, link destinations, widget accessibility
lists	16	L/LI/Lbl/LBody nesting

Use horn list-checks to see all registered checks.

Output formats

• text: Human-readable terminal output (default)
• json: Structured JSON report
• sarif: SARIF v2.1.0 for GitHub Code Scanning
• junit: JUnit XML for CI dashboards (Jenkins, GitLab, etc.)

Test data

Horn uses the veraPDF test corpus (CC BY 4.0) as a git submodule. See tests/fixtures/TEST_DATA.md for details.

git clone --recurse-submodules https://github.com/focusring/horn.git
horn validate tests/fixtures/verapdf-corpus/PDF_UA-1/ --recurse

License

Licensed under the MIT License.