The pdr version has the outer two rows of opens blocked off by rows of dense felt. It allows sound to only pass from the center two rows of openings. This makes the playing surface narrower and improves off axis response. This is on the front side and back side of the diaphragm. It also gives up about 3db worth of sensitivity to the non-pdr version.
So the non-pdr version is better suited as a mid if one were looking for a midrange application.
The pdr version might be better suited for a line source in that it will have better off axis dispersion.
