PD stands for Phase change Dual disk.

The drives were supposed to be able to also read CDs

They were competing with Magneto-optical disks and later CD burners. I liked the robustness of MO but since CD drives were starting to become standard equipment I opted to spend $1200 on a CD burner. Since at that time rewriteable CDs were $25 and a drive to write them was $200 extra I decided to just get the standard burner.