Adobe Reader makes reading fun and colourful.
Functions include highlighting text, striking through, underlining, adding typed text, writing or drawing anywhere on the document, and adding a personlised signature.
I use this app regularly on my iPad to make notes and highlight key points in articles, and I have used it for writing notes on scanned student assignments, so it is as equally useful for learners and teachers.
Available for Mac, Android and Windows portable devices, and desktop computers.