I also sold my iPad 2 and now I don't know what should I do! Wait for the new thinner, lighter, faster etc iPad 5 or go for an iPad 4 which perfectly fit my needs at he moment but will be outdated in 1 month or so?
I can't understand how you think a device is outdated just because a new one comes out. The iPad mini as the same hardware that the iPad 2, and its out for a couple of months only. I would risk to say that the only devices outdated are the iPhone below the 4 and the iPad 1, and just because they get kind of slow when running some new apps and games.
If you use it to study, definitely get an iPad 4. You need the retina display to be able to read PDF without constantly zooming in and out. This is NOT a matter of preference, it's a matter of facts.
iPad with retina display is absolutely the best way to read and annotate PDF files. Better than on any other tablet, better than on any other computer screen, better than on any other laptop, better than printed PDF files.