I am a US Citizen, traveling to France later this year. I have a 7 hour layover in Montreal, Canada. Do I need any other documents, other than my passport when stopping in Canada?
As a US citizen, you should not need anything other than a passport in the situation you described - you can verify this here. Of course, this assumes that the Canadian immigration officer believes you - in case there is any suspicion, be prepared to show proof that your intention is just transit (ticket for onwards travel to France, etc) - but realistically it shouldn't be a problem unless there's something about you that the immigration officer finds suspicious. There may be some special requirements in some special circumstances, e.g. if you have a criminal record you may need additional documents.
For a US citizen no. You need only your passport.