|*.PUB is most commonly a Microsoft Publisher file.|
That said, you'll need Microsoft Publisher (a part of some versions of Office and also for sale separately I think...at least it used to be) to open the file.
There are probably other programs, maybe some free ones, that can open it, but I don't know about them if they do exist. My understanding is Publisher is a fairly proprietary format. Ask your boss if your company can provide you with that software since you need it to work on the flyer.
Free 60 day trial available here:
Note this is (version) 2007, and your file might be an older version. Make sure if you change the file that you save it in the original version if the program offers to save it in a newer version.