PR card is not a Travel document. The only use for PR is when traveling back to Canada,either by land for the CBSA officers or via airports where it's used as proof for the airline staff that you are allowed to Canada and don't need the new form they ask from everyone else(and also via the kiosks).
Try to board any airplane(international) with just PR and you will be refused = not a travel document.
Try to board any airplane(international) with just PR and you will be refused = not a travel document.