This question is asked quite frequently - the last days also by myself 
The answer is that the SCN does not allow to add PDFs, but only the file types referenced in the dialog (like txt, diffrent picture formats, xml).
If you are SAP employee there is an other method to upload PDFs and than to crosslink the PDF by an SCN article.
See also thread: http://scn.sap.com/thread/3465340
The method for SAP employees is to visit page http://scn.sap.com/community/getting-started (tip came from Jason)