How do I control content layout in a WPF documentviewer control?

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WPF

I am trying to use a DocumentViewer control to display a document created on the fly from paragraphs held in the database. I can construct a FlowDocument and display it in a RichTextBox and it works as expected. When I convert the FlowDocument to a FixedDocument and set it as the Document property of the DocumentViewer, it is coming out in two columns, something like a newspaper page. I just want the content to be full, or stretched, or justified, basically no formatting; just use all the width available.
I don't know if it's my code that converts the FlowDocument to FixedDocument that is introducing some formatting, or if it is a property on the DocumentViewer that I need to set.

What I have tried:

I have tried so many online code examples I can't begin to list them. Here is my code for converting FlowDocument to FixedDocument. After that I simply assign the FixedDocument to the Document property of a DocumentViewer.

Private Function GetFixedDocument2(ByVal flowDoc As FlowDocument) As FixedDocument

    ' Create a FixedDocument with the same dimensions as A4 paper
    Dim fixedDoc As New FixedDocument()
    fixedDoc.DocumentPaginator.PageSize = New Size(793.7, 1122.5) ' 210mm by 297mm converted to points

    ' Create a FixedPage for each page of content in the FlowDocument
    Dim paginator As DocumentPaginator = CType(flowDoc, IDocumentPaginatorSource).DocumentPaginator
    paginator.ComputePageCount()
    Dim pageCount As Integer = paginator.PageCount
    For i As Integer = 0 To pageCount - 1
        Dim pageContent As New PageContent()
        Dim fixedPage As New FixedPage()

        ' Set the dimensions of the FixedPage to match the FixedDocument
        fixedPage.Width = fixedDoc.DocumentPaginator.PageSize.Width
        fixedPage.Height = fixedDoc.DocumentPaginator.PageSize.Height

        ' Get the content of the current page of the FlowDocument
        Dim flowContent As DocumentPage = paginator.GetPage(i)
        ' Create a DrawingVisual to render the content
        Dim drawingVisual As New DrawingVisual()
        Using drawingContext As DrawingContext = drawingVisual.RenderOpen()
            drawingContext.DrawRectangle(New VisualBrush(flowContent.Visual), Nothing, New Rect(New Point(), flowContent.Size))
        End Using

        ' Create an Image and set its source to the DrawingVisual
        Dim pageImage As New Image()
        pageImage.Source = New DrawingImage(drawingVisual.Drawing)

        ' Add the Image to the FixedPage
        fixedPage.Children.Add(pageImage)

        ' Add the FixedPage to the PageContent
        pageContent.Child = fixedPage

        ' Add the PageContent to the FixedDocument
        fixedDoc.Pages.Add(pageContent)
    Next

    Return fixedDoc

End Function