What should I use for PDF compliance checks and validation?

It depends on which kind of PDF compliance you mean. “PDF compliance” can refer to several different standards:

• PDF/A for long-term archiving
• PDF/UA for accessibility
• PDF/X for print production
• General PDF syntax/structure validation for file correctness

Good tools by use case

1) General PDF validation / conformance

• Adobe Acrobat Pro — common for manual inspection and preflight
• callas pdfToolbox / pdfaPilot — very strong commercial options
• veraPDF — best-known open-source validator for PDF/A
• Apache Preflight — useful for basic PDF/A checks, but less complete than veraPDF

2) PDF/A compliance

• veraPDF — the go-to open-source validator
• callas pdfaPilot — commercial, widely used in production
• Adobe Acrobat Pro Preflight — useful for checking and fixing
• GWG/Preflight profiles if you’re in a publishing workflow

3) Accessibility / PDF/UA

• PAC 2021 — common accessibility checker for PDF/UA
• Adobe Acrobat accessibility tools — helpful, but not a full validator
• CommonLook PDF Validator — commercial and widely used
• AxesPDF — another commercial option

4) Print compliance / PDF/X

• Adobe Acrobat Pro Preflight
• callas pdfToolbox
• Enfocus PitStop Pro

My short recommendation

If you want a safe default:

• PDF/A validation: use veraPDF
• Accessibility / PDF/UA: use PAC 2021
• General production checks / fixing: use callas pdfToolbox or Adobe Acrobat Pro

If you’re building this into software

• Use veraPDF for PDF/A validation
• Use a parser/library like Apache PDFBox, iText, or PyPDF for structural inspection
• Don’t rely on a generic PDF library alone for standards compliance

If you tell me:

1. which standard you need (PDF/A, PDF/UA, PDF/X, or just valid PDF), and
2. whether you want open-source or commercial,

I can recommend the best exact toolchain.