As far as I know, MS Publisher files can only be opened and edited by MS Publisher.
So if it's only for reading, you'll have to ask the sender to convert it to something you can read.
http://desktoppub.about.com/cs/publisher/f/share_pubfiles.htm has some suggestions.
I recently had a .pub extension file sent to me. What is the easiest way of opening into, say, .pdf or .doc. I have Office 2000 with the Compatability pack for Office 2007 system version 12.0.6612.1000 and Adobe ReaderX. Is there an addon or update that might help withought me buying Publisher?