At work my boss is trying to get people to work more in the digital world and save trees. Is there any setup for my Mac where I could open a document (e.g. Word, Excel, PDF, HTML), and then draw on it like I am taking notes on a piece of paper? I am mostly interested in being able to do things like circle things, cross things out, draw free-hand diagrams, but being able to actually write would be even better. I have a 2012 MBP with Mountain Lion on it. Not sure what hardware or software I would need. Can this be done? Oh, BTW, if there is anything that is open-source, that would be a bonus!