[More recently I haven’t been having this problem. I seem to be able to specify

FontFamily->”Helvetica”, FontWeight->”Light”

and everything works fine.]

I often have trouble when exporting Mathematica graphics to PDF or EPS for later post-processing in Adobe Illustrator (or InkScape). I am not exactly sure what causes the problem, but the incompatibility is font-dependent, so I have this workaround.

I want the final graphics to include Helvetica text, so I choose a non-problematic font that is similar to Helvetica, like “Microsoft Sans Serif”, and use it for Mathematica graphics. The key characteristic of the non-problematic font is that it imports into Illustrator as text (not outlines, not non-printing characters that render as boxes or symbols, etc.). So something like this works:

BaseStyle -> {21, FontFamily -> "Microsoft Sans Serif"}

Then I can replace the temporary font in Illustrator using the Find-Font dialog box.

The bug is really annoying but always shows itself when you are working on a figure and don’t have time to track down exactly what is going on.