Reset File Explorer PDF preview handler

If you're getting 'This file can't be previewed' on PDFs, particularly after installing 3rd party PDF viewers/editors, it may be worth resetting the PDF handler back to the Windows default.

Under the key:

HKEY_CLASSES_ROOT\.pdf\ShellEx\{8895b1c6-b41f-4c1c-a562-0d564250836f}

Set the default value string back to:

{3A84F9C2-6164-485C-A7D9-4B27F8AC009E}