public string GetText(
    PdfDocumentPosition start,
    PdfDocumentPosition end
)

Public Function GetText(
    start As PdfDocumentPosition,
    end As PdfDocumentPosition
) As String

bool mouseButtonPressed = false;
PdfDocumentPosition startPosition;
PdfDocumentPosition endPosition;

void pdfViewer1_MouseDown(object sender, MouseEventArgs e)

{
    if (e.LeftButton == MouseButtonState.Pressed)
    {
        //Retrieve the cursor's initial position
        e.GetPosition(pdfViewer);
        startPosition = pdfViewer.ConvertPixelToDocumentPosition(e.GetPosition(pdfViewer));
        endPosition = null;
        mouseButtonPressed = true;
    }
}

void pdfViewer1_MouseMove(object sender, MouseEventArgs e)
{
    if (mouseButtonPressed)
    {
        //Obtain the cursor's final position
        endPosition = pdfViewer.ConvertPixelToDocumentPosition(e.GetPosition(pdfViewer));
    }
}

void pdfViewer1_MouseUp(object sender, MouseEventArgs e)
{
    mouseButtonPressed = false;
    if (startPosition == null || endPosition == null)
        return;

    //Retrieve the content between two positions
    string text = pdfViewer.GetText(startPosition, endPosition);
    MessageBox.Show(string.Format("You selected the following text:\r\n {0}", text));
}

Private mouseButtonPressed As Boolean = False
Private startPosition As PdfDocumentPosition
Private endPosition As PdfDocumentPosition

Private Sub pdfViewer1_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs)
    If e.LeftButton = MouseButtonState.Pressed Then
        e.GetPosition(pdfViewer)

        ' Get the pointer's initial position
        startPosition = pdfViewer.ConvertPixelToDocumentPosition(e.GetPosition(pdfViewer))
        endPosition = Nothing
        mouseButtonPressed = True
    End If
End Sub

Private Sub pdfViewer1_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs)
    If mouseButtonPressed Then

        ' Retrieve the pointer's final position
        endPosition = pdfViewer.ConvertPixelToDocumentPosition(e.GetPosition(pdfViewer))
    End If
End Sub

Private Sub pdfViewer1_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs)
    mouseButtonPressed = False
    If startPosition Is Nothing OrElse endPosition Is Nothing Then Return

    'Get text located between two positions
    Dim text As String = pdfViewer.GetText(startPosition, endPosition)
    MessageBox.Show(String.Format("You selected the following text:" & vbCrLf & " {0}", text))
End Sub