PIA flight PK305, an Airbus A300B4-203, suffered a rapid decompression at FL280 about 20 minutes after take-off. An emergency descent was initiated. Throughout the descent, control problems were experienced. The crew was able to recover the aircraft at 8,000 feet. The aircraft landed at Lahore safely without injury to pax or crew. Roughly a quarter of the rear pressure bulkhead was found ruptured and blown into the unpressurized area behind the bulkhead. The flight control cables pass through the ruptured portion of the bulkhead. Hydraulic lines behind pressure bulkhead, and APU fuel line also found ruptured. Findings of investigation indicated that the pressure bulkhead failure was due to unnoticed corrosion caused by its proximity to a lavatory.