Uninstall Adobe Reader to begin with
default PDF reader on a Mac is Preview.
It is possible that the Adobe plugin is not working correctly, a problem with Adobe and Lion.
Open the hard drive and go to Library > Internet Plug-Ins and remove ALL Adobe, Acrobat and pdf plugins that you find.
Repeat with your Home > Library > Internet Plug-Ins.
Launch Safari and see if that makes any difference
let us know