Hi,
checker. Is the job a one-off?
No. I would use veraPDF as a validator against a PDF-A3-embedded-XML-file-based invoice metadata standard called ZUGFeRD ( http://www.ferd-net.de/front_content.php?idcat=231&changelang=4 ).
The ZUGFeRD XML structure is more or less schematron based and that's why that particular "twist" as you call it is so appealing.
-> Looks as I would like to contribute something file extractish to veraPDF.
If you gave me a hint how to start writing an according feature extraction plugin I would like to try. And probably fail. And because I love failing, in a second step I would like to fail on a proper veraPDF ZUGFeRD validation plugin ;-)
Let me guess: First step for me would be to try to build https://github.com/veraPDF/veraPDF-plugins/blob/integration/embeddedfileSamp... and then try some amendments?
kind regards Jochen