This property specifies the percentage of boldness to apply when
creating a synthetic bold style.
XPS documents can contain text in simulated bold. Simulated bold
is used where a font does not have a bold variant and a synthetic
approximation is required. However this is a feature which is
specific to XPS and which does not have an analog in PDF.
So this means that during the import process ABCpdf has to
create an appropriate synthetic bold substitute. The XPS
specification says that this should be done using an outline which
is 1% of the size of the font.
However this level of emboldenment can look somewhat lighter
than one might expect. By increasing the value of this property you
can make such fonts look more bold.