I normally prefer Adobe Reader (still v9 on Linux) to all other PDF viewers because I think it has the best rendering quality, even though others have caught up lately. This rendering quality appears to be to a large part due to CoolType, Adobe's implementation of subpixel rendering. Unfortunately I've found that Adobe Reader consistently switches subpixel rendering off (on a per-page basis) as soon as the page contains transparent elements. An example:
Blue shape is opaque, text is rendered using subpixels:
Blue shape is 50% transparent, text is rendered with standard antialiasing:
The two corresponding pdf files have been generated in exactly the same way (using TikZ/PGF with pdflatex), except for specifying transparency in the second case. I made these pdfs myself, but I have frequently observed the same effect with pdfs from completely different sources. Strangely, Foxit Reader and Okular render the same two pdfs in an exactly identical way (except of course for the transparent part itself), but at a generally worse quality IMHO.
My question: Does someone know why this happens, and whether / how it can be avoided?