Anyone remember PagePacker?

https://www.bignerdranch.com/blog/pagepacker-13/

I am an experienced C/C++ developer who has not yet learned Objective C.
I am trying to make time to learn it, but I have something I want to do – I want to expand PagePacker into a more generic PDF Imposition Utility, but I am having a lot of difficulty understanding what happens where in the code.

Thanks in advance.