Because in many cases, the image has been set (inappropriately and unnecessarily) to some printing process density correction which requires to have a minimum of let's say a few percent of black ink in the lightest areas.

Many printing processes can't offer any gradations between no ink and their tiniest dot of ink. So they let white areas be the minimum of a few percent. Otherwise the image would have bleached out areas, with very visible bands of the first possible ink level around it.

Color (and ink) management isn't a matter of simply switching on or off. It constantly needs attention, tweaking, dealing with requirements, and making decisions. The best approach for 'amateurs' (who most of us are) is to just leave the image in RGB (as it was 'born') and let the printer perform its own compensating. Okay, the print probably won't look as bright and brilliant as on the screen. But at least it's the best approximation.


Oorspronkelijk geplaatst op Quora: Why do images with a white background often get this dirty appearance when included on a PDF file?"