There appears to be a funny bug in Preview.app 5.0.3 (504.1):
I took a few PDF pages of normal black text, used Preview's yellow highlight functionality, and printed it on a B&W HP Laserjet.
The pages that don't contain any highlights look normal. But the pages that do are printed in a strange 'jittery' low-quality — all of it, not just the words that are highlighted.
I've seen this happen with a variety of different PDFs that I've highlighted using Preview.
Best Answer
Guessing here... but I suspect that Preview uses a jpg of the pdf onto which it'd add the highlight. The high compression used used by Preview in this process would be where the low-quality comes from.
How to get around it? Use the annotation highlight function in Acrobat Reader instead if you can. Sorry, not a solution but a work-around! Good luck.