PDF/A-3 is a subset of PDF defined by ISO 19005-3 for long-term archiving. Unlike earlier PDF/A variants, PDF/A-3 permits embedding arbitrary file attachments within the PDF container — a capability used by ZUGFeRD and Factur-X to attach a structured XML invoice alongside the human-readable document.
The result is a single hybrid file: open it in any PDF viewer to see the formatted invoice; feed it to an ERP to extract and process the XML attachment automatically.