VeraPDF (https://verapdf.org) is the primary open-source tool to verify PDF/A documents and it's written in Java. It contains a GUI and a command line variant to verify PDF/A documents. LO already includes two external validators for ODF and OOXML, so it would be nice to have additional unit tests for PDF/A and use VeraPDF to check for conformance. This way it would be much easier to write a unit test for PDF/A bugs like bug 136805.
Moving to NEW
Xisco Fauli committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/core/commit/7039373704b2e9c78662c2e44ae81aea522b5f7b tdf#136822: Add VeraPDF as a PDF validator It will be available in 26.8.0. The patch should be included in the daily builds available at https://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More information about daily builds can be found at: https://wiki.documentfoundation.org/Testing_Daily_Builds Affected users are encouraged to test the fix and report feedback.