The latching lugs on the doors are on the inside of the airplane frame. The interior cabin pressure is trying to push the door out, which means the pressure is also pushing the lugs against the frame. Under the full pressurization, there is 8 PSId pushing on the doors, and everything else. So, 8 PSI on a door with roughly 2,000 square inches of area means no one can open a door in flight.
At lower pressurization, maybe it could happen.