PDF thumbnails are not displayed correctly in windows.
If within the Editor Preferences (Ctrl+K) under File associations, changing Preview Handler for PDF Files to Yes, does not result in correctly displayed thumbnails or PDF icons, it may be time to clear the thumbnails cache.
By default, Windows stores the images for thumbnail previews in cache memory so that it can redisplay the images quickly each time you reopen a folder in Windows Explorer (Vista and Windows 7) or File Explorer (Windows 8).
This tutorial will show you how you to clear and reset the thumbnail cache to force Windows to reload the thumbnail picture views into the cache again as needed as you open folders.
Open the start menu and serach for Disk Cleanup then press enter to being running it.
Select the drive you wish to cleanup, in most cases this will be the (C:) drive.
Uncheck everything, and check only the Thumbnails box, and click on OK.
You Will see a prompt asking you to verify if you would like to permanently delete these files, click Delete Files.
Once completed, restart your computer to finalize the process.