A font descriptor can be used by several fonts. However, when first a TrueType font is imported, later trying to import a Cid font leads to InvalidCastException with the message: "Unable to cast object of type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.FontDescriptor' to type 'Telerik.Windows.Documents.Fixed.FormatProviders.Pdf.Model.Elements.Fonts.CidFontDescriptor'." Fix available in R3 2018 SP1 release.