I think it comes down to what CAD and visualisation software you will be needing to run. PDF's are useful for quick viewing of drawings but they should never be used to scale from unless you are positive that no re-sizing has taken place when the file was created.
I am presuming that the iPad would mainly be used for presenting design solutions to clients, lecturers and other students if so there is the
AutoCAD WS app for iPad.
Be aware that a few reviewers would have preferred being able to edit and create drawings with this app, the viability of doing very precise drawings through touch based input rather than with a mouse and keyboard or digital draughting system like a Wacom pad may not be the best solution for this kind of work.
AutoCAD is finally available for OSX so if this is what you will be using at University and in your placements then you are all set. I worked in firms that have used Bentley MicroStation for the majority of architectural design which unfortunately is only available on Windows, but through the magic of bootcamp this is not a big problem!
In terms of what computer tools to purchase if you have the budget I would suggest purchasing a Mac Pro as it will have a longer useful lifespan than almost any other computing tool on the market today, and architecture (as I'm sure you know) is not the quickest profession to get qualified in!

It has far better options for adding additional RAM and graphics cards which will be extremely useful to you in the future as both the software and your design skills put more demands on computer hardware.
When it comes to rendering some beautiful perspectives of your buildings and designs you will be really happy to have the additional power and memory options in a Mac Pro, even previous generations (with intel processors) will be a safer bet for architectural design than the newest iMac or MacBook Pro, keep an eye on refurb models in the Apple store if your budget is not massive.
Personally I'd forget the iPad and get a Mac Pro rather than the iMac/MacBook Pro + iPad combination if it is within budget and it was being used for high level design like architecture.
Good luck with your studies and career, as an architect you will make a big difference to the world and everyone who uses spaces and places that you have helped create. Have lots of fun designing!