I have an Onyx i62HD (6''), but the firmware is almost the same than the one on the Onyx M92, and I think it is very good for reading pdf. I am reading scientific papers on it and even if the screen is small, the experience is very good. You can highlight text, make annotations... And it has a "paper mode" which makes reading two (or more) column pdfs very convenient. Of course, in a bigger reader the experience should be even better, so take a look to the Onyx M92.
