Please allow me to ask one question. You said you want to export a tagged PDF from web application and the tagged PDF does not refer to "Tagged PDF (PDF 1.4) is a stylized use of PDF that builds on PDF's logical structure framework.", right? So I wonder what is your tagging rule?
Besides, compared to word file, PDF may need special permission to read and process PDF. So I think maybe you need to get the permission to access PDF file first. But this is just my personal assumption and I did not know whether it is right.