In a pressurized aircraft it will be vitually impossible to open the door when under pressure. Most on commercial airliners are "plug doors" This means that the door actually enters the aircraft and seats itself against the airframe to plug the hole. It is then latched. I have heard taht on some Airbus aircraft it is possible to open the door under lower pressure differentials due to the mechanical advatage offered by the mechanism. I believe that on Boeings the door operating mechanism would probably shear before opening under a large 1+ differential.