So, Solidworks isn't listening. I know  this b/c I see posts from 2010 and 2011 on the discussion forum re: inability to attach a PDF to the design binder. I recently installed SW2013, and can drag and drop the PDF into the Design Binder, it shows up in the feature tree, but, unless linked to the original file, will not open. This is no good for someone who wants a vendor soecification drawing traveling with the Solidworks file, embedded. I saw  the post about installing the Bluebeam Revu Trial Version and going through those steps. As I was waiting on the trial to download, I thought "HHHmmm..." "Let's try something else". So, although I am disappointed that users are still dealing with this issue for years now, I found an interim solution. Open the design journal.doc located under the Design Binder in the feature manager tree, insert object from within the MS Word environment, using the 'create from file' tab, and selecting the 'Display as icon' check box. This will insert an icon on the page of your file that you can double-clikck to open inside Adobe Reader, and enable the PDF file to travel within your Solidworks design binder, without keeping the PDF file outside of and traveling witth the Solidworks file. Once inserted from within the MS Word environment, you can delete the original file and the PDF will still open from inside the design binder.

Hope this helps!