ZUGFeRD ( Zentraler User Guide Forum elektronische Rechnung Deutschland ) is a open european standard for a structured, hybrid PDF e-invoice . Technically, this works by embedding a XML file into PDF file. Embedded ZUGFeRD files work with all PDF readers but of course these readers may not automatically process the metadata.

At the right you see a sample PDF containing ZUGFeRD metadata generated with the Mustang library. If you open it in Adobe Acrobat Reader just click on the paperclip symbol to see the embedded ZUGFeRD XML structure.
Screenshot of Acrobat Adobe PDF Reader showing a ZUGFeRD invoice with open file attachments tab