Firefox Extensions to PDF annotation

Zotero is a free and open-source reference management software that helps users collect, organize, cite, and share their research sources. It integrates seamlessly with web browsers and word processors to streamline the research and writing process.

Diigo is a comprehensive online tool designed for researchers, students, and lifelong learners to collect, organize, annotate, and share information found on the web. It acts as a powerful bookmarking service combined with robust annotation capabilities for web pages, PDFs, and documents.