Here are the PDF mode's strengths and weaknesses. First the weak side.
This is a diagram of a modest size. To be fair I often have to use different zoom options on my laptop when viewing these diagrams on a much bigger screen. However, reader e-ink display's lack of interactive options (like a mouse pad that could be used for zooming and scrolling) makes it impossible to offer similar zoom capability as on a computer. Therefore diagrams can be difficult to read.
Smaller diagrams with larger elements are not too bad as seen here. So it is not totally useless. You may need to look very closely however. So for my techincal documents I will still rely on a laptop more than this reader.
Next I tried one of those PDF books that are customized for Sony Reader (using the posted Rucker's Postsingularity book from Bookfeader site).
Wow, that is cool. As you can see there are no margins and the text is huge. It is big enough that you can hold the reader as far back as possible and still read without difficulty.
Compare this to the other PDF file. I found this file to be the best balance between screen space use (100%), text size and text content per page (didn't have to scroll as often as for TXT). I am really impressed and will gladly pay for e-books in the future.