A small laptop, perhaps even the upcoming Asus Eee, would certainly be one option. Another is the Pepper Pad 3, which is great for PDFs and e-books (only limited by 7" 800x480 screen), see this recent
pepper forum thread (which started out on performance and ended up on e-books and PDF). If an even smaller screen is ok, then the Everun (4.8" screen and Windows XP) might be an option - although it is more expensive than the already pricey Pepper Pad. Finally, a low cost LCD-based option with a slightly larger screen is the DT 375 tablet, which has been well covered at teleread see for example this
post. Find more posts by searching for DT375 at the teleread main page.