3973 Make Nimbus the default font for PDFs

http://code.google.com/p/otwarchive/issues/detail?id=3973

When we took out `font-family: serif;` to fix the issue with HTML downloads displaying in Batang (a Korean font) in Internet Explorer 9, our server started using a sans serif for PDFs. This made PDF people unhappy. Now we're going to make HTML people who have Nimbus Roman No9 L installed or a serif other than Times New Roman as their browser default unhappy instead.