If you have GoodReader, the iPad's 'Swiss Army Knife', then you can simply copy it via USB or WiFi.
Most Forum members agree that GoodReader is their No. 1 'Must Have' iPad app, because of its very comprehensive and flexible file handling facilities. Although transferring your PowerPoint file via DropBox or other cloud storage is fine, you'll be transferring this over a broadband link, which could be time consuming (see below). The app gets universally good reviews in the App Store, it's very reasonably priced, is well supported and frequently updated with new (user requested) features. It enables you to import *any* file of *any* size and then open it in the compatible app.
Direct transfer using WiFi will probably be very much faster - say 54 Mbps - than broadband - difficult to know how long this would take without knowing what data rate you get.
One issue that you might possibly come up against is that, as you undoubtedly know, Keynote is a 'stripped down' version of the Mac's iWorks program in terms of functionality and capability. I would not be surprised if a PowerPoint presentation of that size doesn't cause the iPad/Keynote to 'struggle' - remember the iPad only has 256 MB of memory (RAM).
Good Luck whatever method you choose. Please let us know how you get on....
Tim
Scotland