Strange enough, the PDF reader icon corrupts in my Dropbox folder on my Windows 8 machine. I am using Windows 8 x64 Pro.
If I change the "open with" to some other reader and then back to Adobe Acrobat Reader, it shows the PDF icon and preview of the PDF file. But somehow it MAGICALLY corrupts sometime later and the PDF file gets a blank icon. Possibly corruption of thumbs.db in Dropbox during synchronization?
This screenshot is in the root folder of my Dropbox folder on my Windows 8 machine.