I am putting together a series of PDF files into one. I am using Adobe professional 9, I create the 1 page file and using the "document/insert pages" command I insert the pages 1 by 1. Sometimes when I try to delete or extract a page I get a box that says "An incorrect structure was found in the PDF fille" and the file freezes and can no longer be used. What causes this error and is there a way to avoid it?