Where is a bank of examples for using each function in PDFKit?

There isn't really a 'bank' of examples. Places like the Big Nerd Code Ranch have Xcode tutorials for creating apps that display PDFs using PDFView. Most of the original Quartz coding books were written before PDFKit was introduced, so use CGPDFDocument instead of PDFDocument, etc.

I've produced a suite of python scripts that use PDFKit to do pretty much everything that PDFKit can do. As well as being functional, they're designed to be exemplars of how PDFKit works. If you know Swift, and are working in Xcode, then it should be easy to translate to Swift.

On github, find /benwiggy/PDFsuite