Join Date: May 2009
Location: 26 kly from Sgr A*
Device: T100TA,PW2,PRS-T1,KT,FireHD 8.9,K2, PB360,BeBook One,Axim51v,TC1000
If your baseline is US$250, then your best bet is a netbook.
You could hunt for a LINUX or WinCE MID (there's a forum here for such devices) or wait a bit to see what comes in the way of tablets in the next 6 months.
But, as pointed out, your focus on pdf as a filter format excludes pretty much all eink devices this side of KindleDX and (maybe) the iREXes.
Proper display of PDF really needs a computer and/or a largish color screen.