How to fix PDF-18
The XMP has no PDF/A extension schema for the ZUGFeRD / Factur-X properties
A check of the PDF of a ZUGFeRD or Factur-X invoice, not of its XML. It runs when you upload a PDF; the embedded XML then gets the XSD and EN 16931 checks (XRechnung: also the KoSIT rules). Check your PDF with the ZUGFeRD validator or the Factur-X validator. What the PDF check covers.
PDF/A-3 (basic check). This is one of a few basic PDF/A checks. They are not a full PDF/A validation: rules such as CIDSet, transparency or tagged structure are not checked. For a full PDF/A-3 check, run the PDF through veraPDF.
What this rule checks
PDF/A allows XMP properties outside the standard schemas only when the XMP describes them in a PDF/A extension schema (pdfaExtension:schemas with pdfaSchema:namespaceURI). The ZUGFeRD / Factur-X specification includes this block; without it a PDF/A validator rejects the file.
How to fix it
Add the pdfaExtension:schemas block of the specification, with the namespace URI of the properties used.
Related rules
Validate your invoice
Upload your corrected invoice to check it passes PDF-18 and all other ZUGFeRD / Factur-X PDF checks and EN 16931 rules. Free, instant, no signup.