The fundamental problem is that complex PDFs need fast CPUs and lots of RAM to render correctly, and at an acceptable speed. eInk devices generally prioritise long battery life over performance, and have slow CPUs and little RAM.
Eg, try this public domain PDF:
http://oi.uchicago.edu/pdf/paintings1.pdf
(It's a scanned book containing pictures of paintings from Egyptian tombs, and is about 140MB in size).
On my iPad, the rendering speed is slow, but acceptable (it takes about 5-10s to render each page with a painting on it). I would hazard a guess that on an eInk tablet such as the T68 the performance would be so slow as to be unusable. I'd be interested to know how it performs, if anyone would care to try it!
This may of course not be an issue for PDFs that contain only text.