Rated 3 out of 5 stars
This works well but for some reason adds &embedded=true&chrome=true to the query, which presents a very ugly docs viewer (missing thumbnails at the left, awkward compact interface, way too much zoom by default, and on top of that chrome=true means you'll end up wasting a lot of vertical space anyway for nothing).
I edited the addon to remove that part of the query sting — much better. Maybe some pref could be added for this. Also, as some people have mentioned, right click on link -> save PDF would be much appreciated. But do note you can File -> Download original from within the viewer.