The dittobook D9 will be out in march. should be
excellent for PDFs but we still havent seen it yet.
Also the Onyx Boox with its zooming selections is excellent for technical papers. some think a 6inch can't do them well but this does show the best way to do them on a 6inch screen. It does let you write on them as well
The PocketBook 301+ is just as good or better than Onyx at displaying PDFs on a 6inch screen and is cheaper than the onyx because it doesnt have that wacom pen input.
the Irex DR800 had the zoom functions its older brother does then it would an excellent choice as well. hopefully they will fix that with an update soon.