Ask HN: How to keep high quailty images when printing to PDF?

I've had this problem for a while, but haven't found any solutions yet. Maybe some of you guys know what to do. Occasionally I need to print a document (usually a .docx) to PDF, and usually I use PrimoPDF to do so. However, whenever there's an image included as part of the document, the image loses significant quality when printed to PDF. Is there a way to print to PDF files without losing the quality?

For example, SEOMoz offers a beginner's SEO guide online. (http://guides.seomoz.org/chapter-3-why-search-engine-marketing-is-necessary) They also offer the same guide as a downloadable PDF. The pdf includes all the images used as part of the guide, but without any noticeable loss in quality. How would I go about doing the same thing?