warningFactur-X / ZUGFeRD containercontainerPDF
A Factur-X PDF must carry XMP metadata declaring the document type, attachment filename and conformance level.
The PDF carries no XMP metadata, so nothing in the file declares it to be a Factur-X invoice.
Add the Factur-X XMP extension schema to the document metadata and produce the file as PDF/A-3. Without it you have a PDF that happens to have an XML attached — which often still works in practice, but is neither Factur-X nor PDF/A-3, and France's mandate expects both.
Nothing is stored, and the whole invoice is checked, not just this rule. A PDF only works if it is Factur-X / ZUGFeRD — a PDF with the invoice XML inside it.