The KDX has demonstrated that a 9.7" EInk screen is big enough for letter/A4 PDFs. However, it has the poorest support for PDF features imaginable. Amazon essentially stopped working on it as soon as they could get a PDF to show up on the screen.
There are probably going to be half a dozen ~10" EInk devices announced at CES in January. If you want something now, then the Kindle DX may be the only option at a reasonable price and it does display PDFs well, but it is going to look very second rate by next spring.