Margin adjustment in PDF

[patrickmj]

Imported from Trac ticket #27 reported by sramsay on 2010-08-07:

PDF allows fine-grained control over margins (and, in fact, can manipulate gutters in points, millimeters, centimeters, or inches).

Ideally, I'd like to figure out a way to set the default margins based on the character width of the text block (in accord with standard typographical practice in book publishing). But at the very least, the user should be able to set these parameters.