The 5"and 6" EInk screens are not large enough for letter/A4 PDFs. A 7" or 8" screen might be big enough, but the contenders (Sony Daily Edition and iRex DR800SG) are not out yet, both are due any day now. A 9.7" screen or larger is big enough. The best EInk PDF device today is the iRex DR1000S, but it is very expensive. The best less expensive device is the Kindle DX, but its PDF support for everything except displaying the pages is very poor (e.g. no TOC).
There are likely to be half a dozen ~10" Eink devices introduced at CES in January. They won't all be for sale in January, but this will greatly expand the PDF-capable Reader landscape.
See
E-book Reader Matrix: Larger Devices for more info.