Private Sub RadPdfViewer1_MouseUp(sender As Object, e As MouseEventArgs)
        For Each visualPage As RadFixedPageElement In Me.RadPdfViewer1.PdfViewerElement.ViewElement.Children
            For Each editorElement As RadElement In visualPage.Children
                Dim tb As RadTextBoxControlElement = TryCast(editorElement, RadTextBoxControlElement)
                If tb IsNot Nothing Then
                    font = tb.Font
                    RemoveHandler tb.TextChanged, AddressOf tb_TextChanged
                    AddHandler tb.TextChanged, AddressOf tb_TextChanged
                End If
            Next
        Next
    End Sub

    Dim font As Font
    Dim width As Double
    Private Sub tb_TextChanged(sender As Object, e As System.EventArgs)
        Dim tb As RadTextBoxControlElement = TryCast(sender, RadTextBoxControlElement)
        Dim textToMeasure As String = tb.Text
        Dim fontSize As Single = tb.Font.Size
        Dim measured As SizeF = TextRenderer.MeasureText(textToMeasure, font)
        width = tb.Size.Width
        If measured.Width > width Then
            Do
                fontSize = fontSize - 1
                font = New Font(tb.Font.FontFamily, fontSize, FontStyle.Regular) 
                measured = TextRenderer.MeasureText(textToMeasure, font)
                Console.WriteLine(fontSize & " >> " & measured.Width & "available " & width)
            Loop Until measured.Width < width
        End If
        tb.Font = font 
    End Sub