Hi RM,
If Document Assembly is shown as Not Allowed in the File-Properties Security tab then the PDF has a Password applied to it. If you know the Password, then you can change the Security settings ( in Acrobat 8 and 9 under the Advanced tab, and in Acrobat X it is in the Tools panel under Protection-Encrypt). If you do not know the password you can't change it.
Hope this helps,
Dimitri
WindJack Solutions
www.pdfscripting.com
www.windjack.com