I don't think that's relevant or true about "his books" in general, as (for instance) the digital versions of TAOCP look great (and use vector fonts). Obviously the publishers will use their own workflow; in that case it's not as if Knuth prepared a PDF for you to download (as he did here). Maybe the publishers just botched up the electronic version of Concrete Mathematics (as the reviews say), or you bought a copy that's defective somehow.