I did cross the boarder only with PR Card. But as per my understanding, sometimes , it is possible that the return to Canada is not permitted with COPR copy.
If you have a US entry visa then you can cross the bridge and enter USA.
After your stay in USA when you wish to comeback to Canada you reach the Canadian check point at Rainbow Bridge or Peace Bridge or else where, as you like. The officer will ask your passport and check your visa status. Because you are a landed immigrant he/she will ask for the PR Card. if you show the COPR Copy, the officer can let you enter Canada or refuse entry, which is according to his/her discretion. if your entry is refused, you have to make a U turn and go back to US.
You can go to the Canadian Consulate in Buffalo and apply for the PRTD to enter Canada without any problem.
About PRTD by Buffalo Consulate General Office:
http://www.canadainternational.gc.ca/buffalo/visas/tdpr_info_tvrp.aspx
Please note that usually, when a Canadian PR cross the boarder by road, the officer at the Canadian entry gate may allow with the copy of COPR, but it depends the situation and mood of the officer.
