When I upgraded to Windows 8 64-bit a few months back, a strange thing happened. The thumbnails for all of my PDF files disappeared. Normally, when I view an Explorer folder, I have the PDF files on "view large icons" (as it then gives me a preview of what the first page of the file looks like). But Windows 8 wiped all of that. I couldn't even get the standard Adobe PDF logo. So I did my usual and went Googling for answers and ended up with a free program called 64-bit Adobe Preview Handler. See all Software reviews.
This small app (which weighs in at a measly 1MB) makes a small Windows registry fix that brings your PDF thumbnails back. The issue of disappearing thumbnails doesn't seem to be confined to just Windows 8 though. Anyone with either Vista or Windows 7 64-bit may also experience this problem, and 64-bit Adobe Preview Handler is here to save the day. See all software downloads.
Simply do the usual download and install and when you start it up, you will see a small window. There is pretty much nothing to configure--no options or anything. Click "apply fix" and then restart Windows Explorer. If it still doesn't work, you may also have to clear your thumbnail cache, but don't worry, this is very easy to do. Simply run Disk Cleanup, select the C drive, put a tick in the Thumbnails checkbox, and then click OK. The system will do the rest.
Check back again on Windows Explorer, and you should see your PDF thumbnails appear again.
You have to bear in mind that this is not an official fix by Adobe, and according to the developer of this fix, Adobe doesn't seem to be in much of a rush to make an official fix. So until they can be bothered (which might well be never), this unofficial fix by Leo Davidson is more than sufficient. And if you like the fix, then support the developer by sending him a PayPal donation. This will encourage him to continue the unofficial work that he is doing on Adobe's behalf.