The "pec deck" machine is not one of my favorites. This is a lever type machine you will find at many gyms. It has a seat and arms for doing chest exercises and developing the chest pectoral muscles. If you don't get form right for this machine exercise it can put your shoulder joint, particularly the rotator cuff, under undesirable stress, which could result in injury. Heavy loads add to the risk factor.
I much prefer standing cable flys. You can get the shoulders and arms in a much safer position. For this exercise, attach cables to a high pulley, stand between them, grasp handles and pull toward the center of the body. You should bend over and forward slightly (with back as straight as possible), so that your hands meet somewhere between waist and groin level. The main thing to note is not to get the arms too parallel across the chest as this is the vulnerable position for the shoulder joint, especially with heavy weights.
See a short video demonstration of this exercise.