The ZUGFeRD and Factur-X Formats for Electronic Invoices

"ZUGFeRD leverages an important feature of PDF/A-3 (ISO 19005-3) which allows to embed attachments of arbitrary types into the PDF/A document"

Oh - you mean I can send documents that have arbitrary payloads to my tax office? That sounds interesting.

For some open source implementations see https://zugferd.github.io/open-source.html