Unable To View PDF Preview In Windows 7 Explorer
If you don’t get a preview of your PDF in Windows 7 Explorer, it may be using the wrong PDF document handler.
In Adobe Reader, under Edit > Preferences > General:
At the bottom of page Select Default PDF Handler.
More steps and info here:
http://www.sevenforums.com/general-discussion/35751-cant-preview-pdf-docs-windows-explorer.html