Lumin PDF is a neat tool that I wrote about a couple of years ago when one of my colleagues needed a way for her math students to draw on PDFs that she sent to them in Google Classroom. You can read more about that situation right here. 

I just received an email from Lumin PDF announcing that all of the premium features are now available to teachers and their students for free! The premium features include merging and splitting PDFs, highlighting (in addition to the normal annotating options), and the removal of banner advertising. And Lumin PDF integrates with Google Classroom. You can register for a free Lumin PDF premium account here.