einvoice-preflight can repair the arithmetic inside a Factur-X PDF and write the file back with an incremental update. Until today its tests only used its own reader to check that result, which proves little: a writer and a reader by the same author can agree on a broken file.
So we ran the repaired PDF through poppler. pdfdetach returned exactly the corrected XML, so the repair itself held. But pdfinfo printed Syntax Error: Invalid page count 0 for the originals as well. The example PDFs, built by a test helper, had an empty page tree, and no viewer could show them.
The helper now writes one page with a line of text, the three examples were regenerated from their own embedded XML, and a test repairs the credit note with defects and checks both poppler commands (it skips where poppler is missing). Not yet done: veraPDF and Acrobat, and a PDF from a real accounting system.
The checker is einvoice-preflight; nothing is uploaded.