I used Word 2007 to create a PDF file with an 1526px * 900px image filling a whole page. This is not the first time it's happened, but Google Docs PDF viewer absolutely mangles the colour rendering making it unusable.
I've taken screenshots at the same zoom level in Google Docs viewer and Foxit Reader.
Here's an image for comparison:
It's awful! I've tried messing about with some things, but can't find anything that can correct this issue.
In Chrome you can select "Print" and then "Save as PDF". The image quality in the saved PDF file will go up significantly, compared to the one from "Download as PDF". Google seems to be optimizing images to preserve bandwidth.