PDR is simply short for Paintless Dent Removal - the art of removing dents, dings, creases, large dents, and hail damage from an automobile without the need for repainting or refinishing.
Mercedes Benz is credited as one of the first known users of the technology as early as the 1950's. In order to avoid having to repaint new cars that accidently acquired small dings or dents on the assembly line, factory workers would use PDR to massage the dents away.
In the late 1980's the technology was expanding to the United States but was rather limited to the size of dents that could actually be removed.
During the 1990's technicians in the industry began to push the art of PDR into new frontiers of possibilties moving from small door dings to larger dents and hail damage.