Like it or not, PDF is a common format for sharing online documents. Unlike Adobe Reader, Acrobat is a PDF creator that also offers a range of features to help teachers and learners highlight, notate, edit, mock up and provide audio or written feedback within the document. I use it daily as a student. With educational pricing, a copy runs between $99 - $160 depending upon the basket of features you want.

p.s. While there are many free PDF creators and readers out there, I have yet to find one with advanced PDF reading and editing functions that allow you to highlight, add comments, etc. If anyone has found one, please share. Thanks!