Correction: last I checked, they didn't write the PDF renderer themselves, it is Google's PDFium[1] which was open-sourced a while ago. Before they were using Apple's built in PDF engine, but apparently that had too many bugs for them.
It does not do Postscript, the "PS" in PSPDFKit are the initials of the creator, Peter Steinberger (really nice guy, by the way!). Two/three letter prefixes are common in Objective-C as there are no namespaces (I use MPW).