You have started an export via Rechnungsdatenservice 1.0. The export of one or more documents fails with the error 'Invalid file'.
One or more files in the export are invalid PDFs. This sometimes happens when PDFs are not properly encoded during creation. Or the files are corrupted for other reasons.
Download the file and check if you can open it at all. Sometimes already just opening or viewing the file does not work. Then the whole file is faulty. In this case, you have to ask the supplier to send the invoice again or repeat the scan.
If you can open and display the file, the encoding of the PDF is probably faulty. You can download the PDF file once and export it or save it as a PDF again. It is also best to let the supplier know about the problem so that they can fix it directly for the future.
Click here for the official DATEV documentation for error #DCO01004 (in German).