What are some alternatives?
Liner - Put simply, Liner is a virtual highlighter. It works on PDFs and webpages in the same way a regular highlighter works on books and magazine clippings. Read more about Liner.
Pocket - When you find something you want to view later, put it in Pocket.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
Raindrop.io - All your articles, photos, video & content from web & apps in one place.
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Diigo - Diigo is a powerful research tool and a knowledge-sharing community