Summary:

What is the significance of all these formats?
Do I have to keep all the documents?
Do all attachments have the same content?

Additional Information:

You receive at least two documents: a structured XML format and a PDF.
The structured format is the legal invoice which was forwarded to us by the sender.
The PDF is a “human-readable format” which has to be created and supplied on the basis of the structured format.

Both documents have the same content and both should be kept together.
The Belgian Ministry of Finance requires that the original invoice (the structured format) together with the readable format (the PDF) are electronically stored for a period of seven years. Depending on the document format provided to us by the sender of the invoice, a third attachment may be added.
This document must also be kept.