Hi all

Well it's not actually an Okular problem but somewhere deeper in the stack. But as Okular is involved you might be interested anyway.

When printing a PDF containing a graphic in png format the print quality of that text is reduced. I realized this issue when I got a new color printer (Brother DCP-9045CDN). Text is printed badly pixelized looking like 150dpi (without hinting). Printing the same text containing a jpg (or without) graphic instead of a png does not show this problem and text is printed just fine.
Printing the original document (incl png) directly from OpenOffice or printing the PDF with png from Windows with Acrobat Reader or OSX with preview works fine as well.

As I'm using Kubuntu I filled a bug at Launchpad: https://bugs.launchpad.net/ubuntu/+bug/694293
There is some more information in the bug report.