The line spacing for the glyphs in this font.
This broadly reflects the distance in 1000ths between the bottom
of the lowest descender and the top of the highest ascender on two
subsequent lines. We follow Microsoft best practice on this metric
and vary the calculations depending on whether bit 7 of the
fsSelection entry in the 'OS/2' TrueType table is set or not.
However it is worth noting that many items of software including
Microsoft flagship products do not.
The PDF format only supports a limited range of metrics so
FontObjects read from PDF may contain some approximations compared
with 'live' FontObjects which have been added using the Doc.AddFont
or Doc.EmbedFont methods.