A 6 inch reader is not really the best option to read techincal PDFs.
I read a lot of Programming PDFs, so I need to keep the original format and layout (no reflow).
I currently use the Sony PRS 505, which is a 6 inch screen like the new kindle 3, but I crop the pages to remove the header, footer and margins. I take the edited PDF and process via pdflrfwin.exe which automatically remove any remain excessive margins.
The resulting output is readable on the PRS 505, but about on average 70% in size of the hardcopy version of the book.
I have a friend whom purchased a Kindle 3 and asked him to test a techical pdf. I did process the techincal pdf to remove unwanted space (header, footer, margins and move subtitles etc).
The kindle 3 did display the context quite well in landscape mode and did autosize to remove any remaining white space (margin etc.) , but I been informed this feature does depends upon your PDF document.
IMHO the kindle DX is the better option, but is not the most cost effective option.
The best PDF reader for portable use is a netbook. You can get a decent 2nd hand netbook for half the cost of a Kindle DX, but battery life can be an issue for some users.
Yes I also have a netbook which I use to read PDFs, but I use the Sony PRS 505 on public transport when traveling to work or when my netbook is charging.