I am in medical school as well! May I ask what your focus is, and what you'll work with after graduating? I bought an app called “iAnnotade PDF“ which does cost a few bucks, but is THE best app for the exact thing you are asking for. I use this app every day in class for taking notes, highlining, underlining, and striking out unnecessary parts of the current lecture. I must say it is an AWESOME experience using iAnnotate PDF! It even has a built in web browser which you can use to download pdf files directly from the internet! In my case, our university has all the current and up to date pdf files which are used in class, available on the internet. I simply use the built in web browser to download them all to the iAnnotate PDF app, and then I'm all set! Check it out and tell me what you think about it!