If you're using Acrobat Distiller to create your PDF output, you may find characters missing. This may manifest itself as messed-up maths equations (missing "-" signs, for example), or bits missing from large symbols. Early versions of Distiller used to ignore character positions 0-31 and 128-159 of every font: Adobe's fonts never use such positions, so why should Distiller?
Well, the answer to this question is "because Adobe don't produce all the world's fonts" - fonts like Computer Modern were around before Adobe came on the scene, and they use positions 0-31. Adobe don't react to complaints like that in the previous sentence, but they do release new versions of their programs; and Distiller, since at least version 4.0, has recognised the font positions it used to shun.
Meanwhile, TeX users with old versions of Distiller need
to deal with their fonts. Dvips comes to our aid: the
switch -G1 ("remap characters"), which moves the offending
characters out of the way. The PDF configuration file
(-Ppdf), recommended
in "the wrong type of fonts",
includes the switch.
The switch is not without its problems; pre-2003 versions of dvips will apply it to Adobe fonts as well, causing havoc, but fortunately that problem is usually soluble. However, a document using both CM and Adobe-specified fonts is stuck. The only real solution is either to upgrade dvips, or to spend money to upgrade Distiller.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=distill-prob