I know that Windows 8 has now shipped, but there are still many, many Windows 7 machines out there, and there is a widespread problem with one flavor of Windows 7 called 64-bit.
As I learned, Windows 7 has trouble with PDF searching. That post has helped many people, but I recently came across some other issues that some people have been having with 64-bit Windows: in some cases, PDF previews and thumbnails do not display correctly.
There is a very thorough listing of the fixes over at pretentiousname.com.
This page contains simple fixes for Adobe’s PDF preview handler and thumbnails on 64-bit versions of Windows.
Before I looked into this, people had been complaining about it for over two years with no official response.
So, if you are having a problem with PDF previews and thumbnails not working, go check it out.