If I have a file in OSX (e.g. a pdf), I can choose how to open it i.e. with the "open with..." command from the 'right click menu'.
Sometimes a pdf is garbled in OSX Preview so I need to open it in Adobe Acrobat.
How do I choose the application when opening a file in UNIX?
the man page for the open command indicates-> open -a applicationname
Depending on which applications, you may need to provide the path to the application's actual executible, but:
open -a "Adobe Acrobat 7.0 Professional.app" /Users/USERNAME/PATH_TO/SOME.pdf
works for me.
(substitute the appropriate information in place of the ALLCAPS words 
)
Terminal lets you drag an item from the desktop to the open terminal window to enter it's path.
read the man pages for things. their very helpful.
in this case-> man open
brings up the man page for the open command. 
Thanks. 