Over,
I too have an 18Mb pdf that doesn't open on the cybook, it just gets overwhelmed.
I am very pleased and surprised the DR1000 was successful. see photos
It does take about 10s or so to render the pages for your 20Mb document, but it works.
The case was an extra, if I was spending that much I though it worthwhile & it works quite well. It is the flip-over and doesn't get in the way. As I say though, you will find you rest this device on a desk to read it as it is heavy, especially when in the case.
Howard