Uses resolver in place of the operating system's fonts (SystemFontResolver), asked after
every resolver registered with UseFontResolver(IFontResolver); null asks no operating-system fonts at all, so
output does not depend on the machine.
public PdfOptions UseSystemFontResolver(IFontResolver? resolver)
The resolver, or null for none.
ISO 32000-2 §9.5 NOTE 5 and §9.6.2.2. The default reads the platform's font directories, lazily, on the first font no other resolver has.