Everything published here is generated by the InvoiceXML API and validated against its declared profile before it goes up. There is nothing to attribute and nothing to sign: use the files in test suites, CI fixtures, demos or documentation.
All samples contain fictional parties, identifiers and amounts. They are structurally real invoices and not commercially real ones.
Available sets
Looking for something else
If you are debugging a specific identifier or a validation rule rather than looking for a file to test with, the reference section has a page per specification identifier and per rule. To check a file you already have, the validator takes an upload and names the profile it detected.