Working at Apryse, creators of:
Xodo a free to use all-in-one PDF reader and PDF annotator/editor for mobile and web.
Apryse SDK a cross-platform PDF & Office SDK (formerly known as PDFTron PDFNet).
iText one of the best-documented and most versatile PDF engines in the world (written in Java and .NET).
PDF.js Express Viewer a free fully customizable and improved PDF.js-based viewer with accurate text selection and searching.
PDF.js Express Plus a modern & commercially supported PDF.js wrapper, with annotations, form filling, and e-signatures.