FBReader on a Nokia N800 or N810 will do word search (but not dictionary lookup). I have not used my Nokia 770 for PDFs, but some have reported that reading PDFs (separate reader) is also ok on this device (others say not), despite its small screen. However, I don't know if you can search in a PDF. There are several threads about N8X0 and similar devices, e.g.
In defence of PPC/PDA/Nokia tablet: ebook readers and
Eink Display's (Sony PRS505) vs. Nokia N800's LCD Display.
You can also use FBReader on an iLiad, but it is more expensive than the 6" EInk devices.