Something to try.
Uninstall Nitro. Maybe a wee bit drastic but it ought to get the job done eh.
Or, can't you select a PDF with the file browser then go to the context menu and get into an "open with" dialog to select Adobe Reader as the default application?
Or, open Adobe Reader and go into the application's Preferences (under "Edit").
Select the Category "General".
To the right, down towards the bottom, click the button "Select Default PDF Handler".
Be well...