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.
Evernote - Bring your life's work together in one digital workspace. Evernote is the place to collect inspirational ideas, write meaningful words, and move your important projects forward.
Paperpile - Clean and simple and reference management for the web. Sync your PDFs to Google Drive and cite your papers in Google Docs.
CherryTree - cherrytree – giuspen
Mendeley - Easily organize your papers, read & annotate your PDFs, collaborate in private or open groups, and securely access your research from everywhere.
Standard Notes - A safe place for your notes, thoughts, and life's work