Your ability to leave the Montreal airport is subject solely to time constraints. You will be admitted to Canada in Montreal, or wherever in Canada you first disembark, so you will not be subject to immigration control between Montreal and Halifax. Your visa will perhaps be checked by the airline to verify your status, but there's no question of being "denied entry" or anything of the sort since you're already in at that point.
You've tagged the question transit, but your transfer in Montreal is not a transit. Transit, for immigration purposes at least, means passing through a country on the way to another country. In this case, Canada is your final destination.
So yes, you are allowed to leave the airport. You will probably get the boarding pass for the flight to Halifax when you get your first boarding pass (for your flight to Montreal). If for some reason you don't, then yes, it would be wise to get it before you leave the airport.