When I’m using an OS X application, and need to refer to the Help menu, I jump for joy when I find access to a PDF user’s manual, and nearly break down in tears when finding the alternative — i.e. something implemented for Apple’s Help Viewer. Given that so many developers build Apple-based help for their applications, there must be some advantages. I would guess these advantages are more aimed at developers, because as a user I’d prefer PDF any day.
OSX Application Help