Adobe Reader is free to use and of all of the tools highlighted in this section, it is probably the most universal. Adobe Reader offers easy access to highlighting and annotation tools, and files can be saved and sent to others who can also view your annotations. You can use Adobe Reader on Windows, Mac and mobile devices.
You may also want to consider using a tool that is already on your computer.
If you need to annotate a web page or work collaboratively in a group, consider using Hypothesis. You can annotate any webpage online as well as pdfs, and you can work collaboratively in private groups.