Comment 29 for bug 344814

cairo-ft-font uses FT_GlyphSlot_Embolden to do synthetic bold.
Bug 490475 comment 5 makes me suspect that the emboldened glyph outlines are not embedded.

There are two fonts embedded, one for each of regular and bold weights

/f-0-0 1 Tf
[<0102>-1<030304050406>-1<0307>]TJ
/f-1-0 1 Tf
0 -1.694444 Td
[<01>-83<02>-84<03>-84<03>-83<04>-83<05>-84<06>-84<04>-83<07>-84<03>-84<08>]TJ

but AFAIK the two fonts might be the same. I don't know whether there is any way to tell the PS/PDF interpreter to embolden one of these by any particular amount suitable to match the metrics from FT_GlyphSlot_Embolden.

The backend used on NT uses GDI to do synthetic bold, so I suspect the same issue may exist there. (If it doesn't then maybe that provides some clues on how to fix this for the FreeType backend.)