What is Android PDK?
The Android PDK is the Platform development kit. This is needed by manufacturers that want to build android powered devices or people building custom roms.
It consists of the tools needed to build the Android OS itself.
From wladimir-tm4pda.github.io: "Welcome to the Android Platform Dev Guide! This guide provides an under-the-hood introduction to the Android platform, and is designed for platform developers and manufacturers building Android-powered devices."
That part in italics answers your question.