If the Text you are wanting to preview has images or text formatting you will need to use a RichTextBox, if it is just plain text then a TextBox will work.
To interact with a WPF RichTextBox use a FlowDocument
<Grid>
<RichTextBox Margin="10">
<FlowDocument>
<Paragraph FontSize="36">Hello, world!</Paragraph>
<Paragraph FontStyle="Italic" TextAlignment="Left" FontSize="14" Foreground="Gray">Thanks to the RichTextBox control, this FlowDocument is completely editable!</Paragraph>
</FlowDocument>
</RichTextBox>
</Grid>
For more info refer to;
The RichTextBox control - The complete WPF tutorial[
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RichTextBox Overview - WPF .NET Framework | Microsoft Learn[
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