The pilot stated that he was descending the airplane from an altitude of 8,000 feet after releasing skydivers. During the descent, at an altitude of about 3,000 feet, the airplane's door opened and contacted the underside of the wing. The pilot slowed the airplane and attempted to close the door, but noticed that the door had warped and that the window was missing. A postflight inspection of the airplane revealed that the window had separated from the door in flight and contacted the right horizontal stabilizer, resulting in substantial damage. Examination of the airplane by a Federal Aviation Administration inspector revealed that the door had been modified with a top hinge for skydiving operations, and was closed by means of two pin-type latches that were inserted into holes located on both fore and aft sides of the door frame. The forward pin hole exhibited no damage, while the aft pin hole was torn, indicative of the latch pin being inserted at the time of the door opening.