Are you sure that the code before attaching the file to the work item is working correctly? Everything before the call to function SAP_WAPI_ATTACHMENT_ADD? Did you debug it, do you get the xstring etc.?
The reason I am asking this is that I just recently saw something similar, and it was easy to blame to workflow part (adding the attachment), but actually the PDF was not generated properly at the first place, so there was actually an "empty" PDF file as an attachment, and the error message was very similar to yours.
If the problem really is with SAP_WAPI_ATTACHMENT_ADD, then I would recommend to use the search. There are lots of examples how to use that function.
Regards,
Karri