can anybody explain to me about PD in PA Design?

what is the meaning of "PD" you mentioned above?

He wants the 411 on the pd asap! No bs!

Phase Distortion could be cancelled using a Predistorter.

Phase distortion: Distortion that occurs when (a) the phase-frequency characteristic is not linear over the frequency range of interest, i.e., the phase shift introduced by a circuit or device is not directly proportional to frequency, or (b) the zero-frequency intercept of the phase-frequency characteristic is not 0 or an integral multiple of 2π radians.