Unfortunately, there is no built-in way to assign a specific printer to a
document in Word. If you would care to post through
http://www.microsoft.com/office/community/en-us/default.mspx?dg=microsoft.public
.word.pagelayout&lang=en&cr=US,
you can post a message as a "Suggestion to Microsoft," and users can vote on
it. Needless to say, you will not be the first to request this feature!

As Suzanne has indicated, you cannot change the printer just for Word. You
can change the active printer on Word startup, but this will affect Windows.
You can change the active printer manually from the print dialog without it
changing the Windows default, but on using macro code to do this the Windows
default is changed also. You can print to a particular printer by using
macros - and this may be the best option - see
http://www.gmayor.com/fax_from_word.htm which, despite the page url,
demonstrates how to do this