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