This happens a lot with scanned and uploaded documents. Some scanned documents that are saved to PDF, don't adhere to the document format standards for PDFs. This causes the program that compiles all the PDFs into one document to throw an error.
The work around for this issue is to "un-check" all the items with an attached document and do one of two things:
- Test them one by one to see which one(s) is/are generating the error, then download those attachments separately
- Download all the attachments separately as their own PDFs and include them the print all PDF later