This isn't really a font substitution problem. Currently we don't adjust the widths of characters when we substitute fonts. xpdf has a hack that compares the widths of the 'm' characters and adjusts the size to match. That hack was removed from poppler when the fontconfig patch went in.
This isn't really a font substitution problem. Currently we don't adjust the widths of characters when we substitute fonts. xpdf has a hack that compares the widths of the 'm' characters and adjusts the size to match. That hack was removed from poppler when the fontconfig patch went in.
I'm not sure the best way to fix this yet.