You might want to try command-tabbing and quitting. Basically with command held down the entire time, hit tab until Preview is framed, and instead of releasing

You might want to try command-tabbing and quitting. Basically with command held down the entire time, hit tab until Preview is framed, and instead of releasing command (which would pull Preview to the foreground), hit Q, which will send Preview the command-q event and cause it to quit.