I read 300+ page, heavily-illustrated PDFs on my iPad 1, with 256mb RAM. I wouldn't say it's a perfect experience, but it's perfectly do-able. That makes me think 1gb should be fine for a PDF twice that size.
Of course, someone else will counter that their iPad Air with 1gb RAM crashes constantly, and there's no reason to doubt it. There are too many variables in computing device usage to isolate this kind of thing to RAM alone - number of running apps, background processes, buggy code, etc. Any computing device, up to the largest, fastest supercomputer, can be overburdened. No mortal has build anything that has infinite capacity.
I'm quite confident that, if Apple does introduce split-screen multitasking in iOS 8, that it will run decently on 1gb RAM (and run even better on 2gb, of course). Apple has a long history of making major new features available to several generations of devices. It's just good business - if current owners can't have the newest goodies (within reason), then they're not particularly enthusiastic evangelists for the product. Is there anyone here who thinks that "You have to buy a new iPad to use split-screen" would go over well, with anyone? Why, then, would Apple do that?
When it's obvious that the new feature requires new hardware (front and back cameras added to iPad 2 in order to have FaceTime, Retina-quality graphics, Touch ID sensor...), people understand. When it simply requires a bit more RAM, nobody will be understanding - people have been moaning about RAM bloat for decades.
Apple's been careful to keep a lid on iOS RAM. They'd rather discipline their developers than disappoint tens of millions of end-users (or get into an arms race with a competitor that makes its own chips). And if Apple's message at WWDC is, "If you want your apps to operate in split-screen, you'll have to tighten-up your code," the developer's response is going to be, "Fine, as long as we can sell our split-screen-capable apps to a large customer base."
