It doesn't sound very safe and the draft I found online only describes various restrictions on the PDF and the expectation that the XML is to be seen as an alte

It doesn't sound very safe and the draft I found online only describes various restrictions on the PDF and the expectation that the XML is to be seen as an alternative representation for processing. Nothing to enforce that the contents of both are identical or tamper proof. At best you could claim that an invoice were both don't match is invalid, however that would require manual verification. Meanwhile I write the documentation of my xml configuration files in xsd and convert them to something readable using xslt. One set of data for processing and display everywhere and one less headache about duplicated and badly maintained data.