1, represents a different type of computer:
PDA, Personal Digital Assistant, refers to the handheld computer.
PAD, Portable Device, portable device, now basically refers to the tablet computer.
2, support for different systems:
PDA Pocket PC, support for Windows, Linux, Android and other systems.
PAD Tablet PC, support iOS, Windows, Android and other systems.
3, the use of different purposes:
PDA is mostly industrial-grade PDA, there are also some consumer PDA.
PAD is basically a consumer tool.