I can't speak on the drawing but as far as PDFs go my iPad mini Retina works great for textbooks. I have eBooks on it for all of my classes. Yes the battery life is also great as I only have to charge it maybe once every other day.
charge it every 2 days?!?!? sounds like someone needs to hit the books! just kidding.
but the ipad (waiting to switch to ipad mini) is basically the only tool i use for school. i can either use the computers at school to do miscellaneous stuff (if i need to save an audio/video lecture to dropbox, etc...) OR i just do that stuff whenever i get back to my laptop.
but, my ipad 3 currently has games, productivity apps, but also recorded audio lectures (about 200 hours), recorded video lectures (~200 hours), like 200+ pdf textbooks (most would be over 1000 pgs), and like every set of lecture notes with my own notes handwritten & drawn diagrams over them (annotation via goodnotes) from the last 2 years or so. my notes/textbooks are also bookmarked and searchable.
i can also quickly open wiki/google when i need to quickly & quietly clarify something during lecture/rounds, i can use the ipad to take pics/video of diagrams written on the board that may be difficult to represent in the notes (for example: a lot of teachers make last minute changes to their slides and people dont get a copy of the updated version...so while others are somehow trying to frantically represent the new info into their notes, i can easily paste a pic INTO that exact spot of my notes where it should have been anyways)
its even eliminated needing to pack the backpack between school days & saved carrying around tens of pounds of notes, books, spiral notebooks, etc...
i like the fact that i walk around with that accumulation of knowledge of stuff i already combed thru and have experience with (instead of trying to find a specific thing of info on the internet that i have to re-evaluate & re-process).
hope that makes sense! btw, i dont use a keyboard w my ipad at all...just a cheap ebay microfiber stylus & i handwrite/draw everything.