I have a few 600-700 page PDFs (all text, so the files aren't huge) and they pretty much load instantly. (*) I'm going to go out on a limb and say that, unless you have the attention span of a hyperthyroid Jack Russel Terrier who has just downed a gallon of espresso, the Nexus 7 isn't going to have "too much lag" on much of anything that's stored locally. Most of what little lag I see is when I try to access large files over WiFi.
Of course, you can always pick yours up at a store with a reasonable return policy (I generally buy stuff like this from Staples, for example, because they have a 14-day no-questions-asked full refund policy on hardware). Load your PDFs on day one and run it through its paces. That will also allow you to see if the screen is large enough to view what you really want to view, and upgrade to a Nexus 10 if it isn't.
(*) EDIT: I noticed that someone else mentioned using the built-in PDF viewer, so in the interests of full disclosure I should mention that I use the Kindle app to read PDFs that I'm going to read often because it's good about keeping my place in the document, etc. These are not Kindle books, they are PDFs I have of documentation, etc. I don't generally load large PDFs into Adobe's viewer, so I couldn't tell you much about that.