There's a DPI setting somewhere (I forget where) for PDF output. I think it's High (600), Medium (300), Low (150).
It turns out that what this actually controls is, mostly, the width of hairlines printed to PDF. I found that you should not set it to anything other than the default of 300, or you'll get subtly worse output. If you are really seeking wider hairlines, you could try setting it to 150 and see if you like that.