Ghostscript: not only a PDF/A converter but a C++ library with full Factur-X/ZUGFeRD write supportveröffentlicht von usegroup am 07.03.2021
Ghostscript is one of the very few open source software packages that can convert most, if not all, "ordinary" PDFs into PDF/A.
Apart from the command line software Ghostscript is also available as C++ library. There are a couple of inofficial wrappers, e.g. into the .net domain.
Some of you might have noticed that already five years ago, on Christmas Eve 2015 it has added full ZUGFeRD support with a innocent looking PDFmark function, called Ext_metadata. Documentation as how to use it in conjunction with Factur-X/ZUGFeRD with was a bit sparse in the beginning but apparently in October 2020, they published a dedicated documentation page.