Save as PDF from Word makes pixelated PDF in Acrobat but not Preview
Well, OS X has built in font-rendering that Preview uses, which tends to look pretty great.

Adobe Acrobat must be using their own custom software to render fonts that isn't as good.  Unfortunately, it is up to Adobe to fix their font rendering.  You might want to look for an Acrobat setting that enables anti-aliasing which might help.