There is a definitely a problem with the plugin since couple a days ago.
In "Google Standard Viewer" option it doesn't show anything, and in "Enhanced Viewer" it shows only the doc-viewer frame, but not the PDF document itself. This is an issue in Safari on Mac, and IE9 on PC.
But in Firefox on Mac it works just fine (????!).
I tried several tricks, reinstalled the plugin, change all the possible settings, but it's all the same.
I also tested lots of other links (people that use Google doc viewer posted their links in this forum) and it is always the same: it is not working in Safari (doesn't show anything), but it works fine with Firefox (both on Mac).
So there IS a problem... somewhere out there :)