Need to display very complex PDFs with good performance?
Consider your prayers answered:

SimplePDFKit

A performant, PDFium-based iOS PDF rendering library for single pages.

The PDFium build used is from paulocoutinhox/pdfium-lib, who managed what I never could: to get PDFium to build for iOS (thanks so much!).

Pros

• It's fast. Like really fast. Many PDFs that are barely viewable with the built-in PDFKit are no problem at all for this library.
• Supports virtually any zoom level (well, any zoom level UIScrollView supports; all I'm saying is this library isn't going to be the limiting factor).
• Has simple but powerful APIs for overlaying things onto the PDF, as well as changing the overall visuals.

Cons

• While PDFium is very good at rendering curves, it's actually less performant than the built-in PDFKit when it comes to rendering font-based text! It's not terrible, but definitely noticeable.
• Contains 42 MB PDFium binary. This is really not a big problem though, especially considering bitcode + app thinning will trim it down even further for end users—12 MB of that is x64 rather than ARM, for the simulator.