Best iPad Apps for (Medical) Lectures/Anatomy Revision?

So after wasting about 20 trees-worth of paper and gallons of printer ink in my first year of medicine I've decided it's time to enter the 21st century and get an iPad rather than printing my notes. I'm aiming to get the iPad Pro 9.7 with the Apple Pencil so I can annotate lecture slides but I've really struggled to find a good source online with the most useful applications for medical students. After playing around on a friend's iPad for a few minutes I found I wasn't even able to download powerpoints from my medicine portal and frankly was lost on the App Store trying to find apps for lecture note organising, annotating, etc. Basically, I'm useless.

So tl;dr, can anyone enlighten me on what apps people use to annotate and save lectures? A really good app(s) for each of the following points would be great (whether the app is free or not isn't an issue):

1) Annotating lecture slides (powerpoint, word and pdf - I don't need to record the lectures, just annotate them)
2) Organising/saving lecture files for each module of my course.
3) Anatomy revision/a really good 3D anatomy app.

How do people download powerpoints to their iPad storage because when I tried to do it there was no 'Save' option and it would be a bit pointless getting an iPad if I couldn't even download my lectures to it.

Also, I'm torn between getting the 32gb or the potentially unnecessarily large 128gb model. If I'm downloading 200+ powerpoints would I be foolish not to invest in the 128gb model or should I invest in the 32gb and get cloud storage if I need more space instead? I don't plan to download videos or have copious amounts of pictures - just lecture notes and a few hundred songs, so I'm leaning towards the 32gb model. I'm pretty tech-savvy in general but when it comes to Apple I'm literally clueless as the only Apple product I own is a broken 2nd generation iPod Video.

I appreciate the help and hope this is the right forum to post this in! If not, please can a moderator move it to a more suitable one. Thanks