I would not drop any serious scratch on a dedicated PDF viewer. Instead, why not get a cheap first generation iPod Touch (~120 bucks on CL), or a netbook (~200 if you wait for a deal)? Both options let you do something beyond just read PDFs.
Just get a cheap old windows 98 laptop off ebay, and install a small linux distro, or strip down '98, and install a lightwieght pdf reader viewer software. I myself use both this and my hacked psp to read pdf's.