I disagree. I use mine extensively to read PDFs, technical ones quite often. In fact, I bought a new O'Reilly Head First book (graphics intensive, and only supplied as PDFs) last night and am in the midst of reading it.
I do agree the text is a bit smaller than ideal (i.e. your average hardback) but it is very readable, and not as in "squint and hold the book two inches from your nose" readable. I've read printed texts that way, and the Kindle DX is better than that.