finer control of Preview anti-aliasing (font smoothing)?
I have found a solution to this problem: replacing Yosemite's PDFKit with Mavericks' PDFKit. As simple as that. I replaced the contents of /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/PDFKit.framework/Versions/A and everything is working fine now.

For those who don't have a copy of Mavericks from where you can take the PDFKit files, I have uploaded a copy to: http://www.filedropper.com/pdfkit (hope there are no copyright issues with this - I will remove the link if asked to do so).

By the way, changing the Current symlink to point to the Mavericks version and keeping the original A/ directory DIDN'T fix the issue. I had to remove the Yosemite version completely and put the Mavericks version contents inside A/ (it seems Preview and Skim both are hard coded to Versions/A and do not follow the Current symlink).