Welcome - but maybe you can't afford it
. The PDFs and JPGs aren't ebooks; they're... both image formats (although PDF's can at least be vector images and have links / tables of contents). An ebook is something that can be word-wrapped, has not been split up into pages, has an electronic table of contents, and might even be read by a speech synthesizer.
You can't scale down an A4 page to a 5", 800x600 device. And a digicam of a 5" book at 1:1 is likely to suffer on e-ink: even relative to an equivalent 800x600 PC display, you're going from ~256 levels of gray to ~16 levels of gray. On a good PDF reader, you can use a "landscape" orientation, so you can view an enlarged _part_ of the page - but it gets less feasible as you leave the realm of simple linear text. You're going to curse reference PDFs that don't have an electronic table of contents on e-ink, because page-turns are so slow. And any size of diagram is going to be an exercise in frustration.
I would try waiting for the *official* ("Honeycomb") Android tablets. No guarantees... but if they're popular, they'll keep the minimum price relatively low & include smaller (5") budget models with Wifi only. You'll be able to get more value from it (web browsing), an open system will be better for viewing random filetypes; and the much faster response time, more levels of gray, and actual colour of an LCD screen will be better for image-like formats and diagrams.
It also means you can legally read *both* Amazon and Adobe "protected" ebooks, which covers all the current favorites. (Apple don't count... apart from iBook's device-only reading being a bad choice for a previous PC or Mac reader, they're excluded by your budget. And I suspect their ideas about sync-ing using iTunes would not suit either).