Yes, indeed this is my experience also. I posted about this in the "gray shades" topic. Hope you don't mind if I quote it here:
2-gray-scale setting on the 505?
One source I saw this week (not at hand) referred to a "2-gray-scale mode" on the 505 as an alternative setting, but of course I see nothing like this in the menu.
I was attracted by this, because as I read PDF files on my 505, I notice that the background white is actually one tiny shade off the "true" white of the reader - which is still visible in a strip around the perimeter. Setting the machine for 2 gray scales would solve this problem, it seems to me, and restore the crackle to those PDF pages.
This comes up with every PDF I read, and is a problem absent from other formats on the machine.