With a Hypothes.is user account, you can now navigate IDEAS and highlight text and leave comments for yourself or others. The service has three modes: private (annotate just for yourself), private group (for a research team, a reading group, or a classroom), or public (anyone can see your annotations). Annotations can include tags, links, images, and even LaTeX formulas. Moderators check on comment suitability.
The Hypothes.is functionalities can be accesses though browser plug-ins (Google Chrome, others), a proxy site, or through integration in a learning management system (LMS). All use is free, more advanced features or support are available for a fee. Here is a quick-start guide.