I can't quit Preview. By "can't" I mean that File > Quit is grayed out and the application (which seems to be working fine) isn't in the list for Force Quitting. I can close windows, but I can't actually quit the application.
I've had to force-quit preview a lot recently. Until now, it's been listed for force-quitting and asserts that it's working just fine. Usually, I resort to this when I try to open a new file and it is "thinking" for, well, forever rather than opening the file (a state that renders all previously open files unreadable, because they're grayed out to make the spinning second-hand thing more visible. It has happened enough dozens of times times that I don't think that a corrupted PDF is a plausible explanation. (Large ones might be, though.)