"This file cannot be found" error.

Folks:

I have a user who all of a sudden yesterday started getting an error message every time he tried to open any PDF:  "There was an error opening this document.  This file cannot be found." 

The file CAN be opened using Windows 8's PDF viewer.  It CAN be opened if he opens it from within Acrobat (i.e., File | Open).  But when he double-sclick the icon in File Explorer, he gets that error message.

I tried Repairing the installation, but no success.

Any thoughts?

Thanks.

Micah