How you can help


If you are a developer and you know what to work on we are happy to assist. If you don’t know what to work on, or where to start, or if you want to improve your development skills, please feel free to apply for a open source mentorship.

If you can’t develop you can still


When we change Mustangproject we run the automatic tests and usually at least ZUV to confirm the output remains valid.

Other validators include

  • 3-Heights™ PDF Validator Online Tool is a general (but very sophisticated) validator against the PDF/A-Standard and the
  • GEFEG which runs check on ZF1 as well as PDF
  • VeraPDF (only PDF)
  • Konik (only XML, only ZF1)
  • FNFE’s Factur-X Validator (online, repo)
  • as well as XRechnung (only XML after attaching

    to GuidelineSpecifiedDocumentContextParameter/ID

Internal architecture

  • You provide the human readable representation, i.e. a PDF/A-1 file
  • We use Apache PDFBox to convert it to PDF/A-3
  • You specify the details about the invoice, e.g. the items and their prices
  • Mustang creates a XML file,
  • attaches it to the PDF/A-3 file and
  • e.g. adds a according PDF/A Schema Extension to the file labelling it a ZUGFeRD/Factur-X invoice

We need money for a booth on the annual ZUGFeRD conference and for the development of our opensource ZUGFeRD validator.


ZUV (ZUgferd and VeraPDF) is a ZUGFeRD Validator, released into public beta on 27 April 2018.

It’s a command line tool but FerdMC published a online version on their ZUGFeRD community page.

ZUV will be integrated into Mustangproject in 2019 or 2020 and checks PDF/A-3 compliance (based on VeraPDF) as well as ZUGFeRD 1 and 2 XML correctness. The XML is checked with the official ZUGFeRD schematron and schema files.

There are other open source validators like Konik (which does not check ZF2 nor PDF) or the FNFE Factur-X validator (online-version repo).


Corpus is a (Mustangproject-indendent) collection of freely accessible ZUGFeRD files, some of them found in the wild, some of them crafted. It contains valid and accidentally invalid ZF1 and ZF2/Factur-X files files in different profiles as well as specially crafted invalid files.


  • valid and
  • accidentally invalid files can be used to test one’s import, the
  • files which were build invalid on purpose can be used to benchmark validators.

Feel free to submit files, in particular if you are using a ZUGFeRD engine which is not yet listed.