I have Kindle 3 and for me it's almost useless for documentation PDFs. It is acceptable only in very rare cases; basically once you need to zoom it's a pain. It does display all PDFs I have so I can look up something if I really want it but for "real life" forget it.
In the end you need something bigger and with more pixels, Kindle DX maybe, a netbook (maybe one that is also a tablet, there were some from Asus, Lenovo), one of the bigger tablets or anything similar.