Ghostscript
Not only is Ghostscript a great converter to PDF/A they also added full Factur-X/ZUGFeRD write support on Christmas Eve 2015 (the patch , the documentation to the new function, the sparse initial documentation in form of an issue). In October 2020 they now also documented that in more detail.
Mustang server
I tried to outline the reasons
for and capabilities of the Mustang REST API : Spring boot
made it really easy to just publish the functionality including a
auto-generated swagger API description file.
Quba
Since Factur-X can be displayed with PDF viewers anyway, present
german offline e-invoice viewers like OpenXRechnungToolbox
or Ultramarinviewer
tend to focus on XRechung. The Prototype Fund, i.e. a
collaboration between DLR, OKFN and BMBF, is now funding my project
for a e-invoice viewer named "Quba-Viewer"
from March 2021 to September 2021. "Quba-Viewer"
will also allow to compare Factur-X PDF against it's XML content.
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