This can be most annoying. You’ve found a great article and now you want to download it and read it… but for some reason all you get is a grey screen and a spinning wheel going around, and around, and around (enough)!
Help is at hand. Click on the downloads button (next to the Google search box). You’ve probably already seen a big green arrow pointing down telling you the PDF has been downloaded. OK, click it and this pop-up window should appear with the much desired PDF. Click the PDF and it should open in Adobe Acrobat Pro (or Reader).
Fixing the Problem
This does seem to be a Firefox problem. Once you have a spare minute or two try this for a more permanent solution.
Tools > Options > Applications > scroll down to Portable Document Format (PDF) and select Use Adobe Acrobat in Firefox > Click OK and exit Tools. You shouldn’t have any problems from now on.
Firefox > Preferences > Applications > scroll down to Portable Document Format (PDF) and select Preview in Firefox and that’s it.