Propriedade FlowDocument.FontStyle

<FlowDocumentReader>
  <FlowDocument
    FontFamily="Century Gothic"
    FontSize="12"
    FontStretch="UltraExpanded"
    FontStyle="Italic"
    FontWeight="UltraBold"
  >
    <Paragraph>
      Any font settings on this paragraph would override the font settings
      for the FlowDocument.
    </Paragraph>
  </FlowDocument>
</FlowDocumentReader>

FlowDocument flowDoc = new FlowDocument(new Paragraph(new Run("A bit of text content...")));
// Set the desired column gap to 10 device independend pixels.
flowDoc.FontFamily = new FontFamily("Century Gothic");
flowDoc.FontSize = 12.0;
flowDoc.FontStretch = FontStretches.UltraExpanded;
flowDoc.FontStyle = FontStyles.Italic;
flowDoc.FontWeight = FontWeights.UltraBold;