Want to draw, highlight, underline on a PDF? Try Kami
Kami is a site that allows you to draw, write notes, highlight on your PDF for free. While there is a paid subscription available to the user (allows many helpful additions like being able to annotate on a Word or PowerPoint file), if all you would like to be able to do is highlight, write notes, and circle important parts in your PDF the free version would work quite nicely.
Embedded below is a GIF followed by a 2 minute and 34 second YouTube tutorial.