PSPDFKit

PSPDFKit is the leading framework for displaying and annotating PDFs in your iOS and Android apps and even on the Web. It supports blazingly fast PDF viewing, adding annotations, and filling out forms. Their battle-tested code is trusted by Box, Dropbox, Evernote, and hundreds of other companies. Benefit from a solid, tested codebase and incorporate all those advanced features into your own application. It'll save you at least one year in comparison with in-house development.