The cabin door opened in a PA28 early on in my PPL training in 1975. The instructor, Cliff Barnett, got me to reduce to just above stall speed with flaps down but still couldn’t close the door due to the airflow. We returned to the airfield and I landed successfully while Cliff held on to the door. It was suggested to us by the CFI that opening the vent hatch would have allowed the door to close.