Depends on how detailed the pdfs get. If they have multi-columns, graphs, tables, etc..., that would fall in the realm of a larger screen device. It also depends if those pdfs are image based, or text based. Open one in Acrobat Reader and try selecting text. If you can't, it's likely image based and you would only be able to zoom those on readers with that feature, like Sony. If it's text based, reflow can help by increasing the font size, but it often destroys formatting and pictures.
Large screen devices:
iRex 1000
iRex 800
Amazon Kindle
Sony PRS-900 (large~ish screen)
Here's a thread that may give a bit of help regarding tech manuals on a 6" screen device.
Before picking up my PRS-600, I took some manuals for my HP 50g calculator with me on an SD card and gave it a try. Nothing too complicated in the manuals, but it worked out for me quite well.