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".