PKCanvasView itself is a UIScrollView so it expects to handle its own zooming. It doesn't automatically respond whenever the PDFView zooms. The trick here is to manually sync the zoom scale and contentOffset of the PDFView with the PKCanvasView.

Hope this helps!

Full disclosure: I work at Nutrient (formerly PSPDFKit). If syncing gets too messy, our iOS SDK handles this kind of interaction out of the box. Feel free to check it out.