The helicopter pilot reported that he did a low-reconnaissance flight over the intended landing site, just to the east of a building. Shortly after, he realized that the landing site was changed to the west side of the building. He maneuvered the helicopter to a hover over a new landing site west of the building, about 75 feet above the ground, and started a vertical decent. During the decent, a company mechanic seated in left seat of the cockpit told the pilot of an obstacle to the left of the helicopter. He acknowledged seeing the obstacle and continued to descend. Shortly after touchdown, as the pilot reduced the throttle setting to idle, the main rotor blades struck an empty tractor trailer, the helicopter yawed, and the tail rotor blades struck the trailer. The helicopter sustained substantial damage to the main rotor and tail rotor systems. The director of operations reported that there were no preaccident mechanical failures or malfunctions with the helicopter that would have precluded normal operation.