A recent Gmail update changed the "View" links for PDF attachments, but only if you use Google Chrome. Instead of opening PDF files using Google Docs Viewer, Gmail now uses the PDF plugin included in Google Chrome. Unfortunately, this makes it more difficult to save PDF files to Google Docs.
Here's a simple trick that lets you open a PDF attachment in Google Docs Viewer. Click "View" next to the attachment and edit the URL: replace "view=att" with "view=gvatt" in the address bar. Another option is to right-click "View", copy the URL, paste in the address bar and replace "view=att" with "view=gvatt".
Obviously, you can also disable the built-in PDF plugin. Just type about:plugins in the address bar and click "Disable" next to "Chrome PDF Viewer".