05-28-2014, 01:06 PM
(Tested under Mac OS X 10.6.8)
The Help app will do this.
When viewing the PDF in Preview, choose "Preview Help" from the Help menu.
A floating Help window will come up which will always be the top window covering anything beneath.
This Help window will remain the top window even when you switch to other apps.
Since the Help app is not specific to Preview, you can invoke it from any Mac OS X app that has corresponding item in its Help menu.
The Help app will do this.
When viewing the PDF in Preview, choose "Preview Help" from the Help menu.
A floating Help window will come up which will always be the top window covering anything beneath.
This Help window will remain the top window even when you switch to other apps.
Since the Help app is not specific to Preview, you can invoke it from any Mac OS X app that has corresponding item in its Help menu.