The decision
The sanctioned framework wins.
For a macOS-native target, Apple's PDFKit is the purpose-built framework for rendering, annotations, page manipulation, search and forms. It gives native performance and a small footprint. Reimplementing PDF parsing or shipping a browser engine would both be strictly worse here — so neither was built.
Verified headlessly
A self-test covers blank pages, annotations, flattening, encryption, page ops and reorder — no GUI needed.
OCR under test
A separate harness takes an image-only PDF through OCR and checks the searchable result.