public PdfTextMarkupAnnotationFacade AddTextMarkupAnnotation(
    IEnumerable<PdfQuadrilateral> quads,
    PdfTextMarkupAnnotationType style
)

Public Function AddTextMarkupAnnotation(
    quads As IEnumerable(Of PdfQuadrilateral),
    style As PdfTextMarkupAnnotationType
) As PdfTextMarkupAnnotationFacade

public Form1()
{
    InitializeComponent();
    pdfViewer1.LoadDocument(filePath);
    pdfViewer1.MouseDown += pdfViewer1_MouseDown;
    pdfViewer1.MouseUp += pdfViewer1_MouseUp;
}

PdfDocumentPosition startSelectionPosition;
void pdfViewer1_MouseDown(object sender, MouseEventArgs e)
{
    // Get the start position of the selection
    startSelectionPosition = pdfViewer1.GetDocumentPosition(e.Location, true);
    if (startSelectionPosition == null) return;
}

void pdfViewer1_MouseUp(object sender, MouseEventArgs e)
{
    // Retrieve the end position of the selection
    PdfDocumentPosition endSelectionPosition = pdfViewer1.GetDocumentPosition(e.Location);
    if (endSelectionPosition == null || startSelectionPosition == null) return;
    if (endSelectionPosition.Point.Equals(startSelectionPosition.Point)) return;

    // Define the points to build a quadrilateral
    PdfPoint p1 = startSelectionPosition.Point;
    PdfPoint p4 = endSelectionPosition.Point;
    PdfPoint p2 = new PdfPoint(p4.X, p1.Y);
    PdfPoint p3 = new PdfPoint(p1.X, p4.Y);
    int pageNumber = startSelectionPosition.PageNumber;

    // Create a list of quadrilaterals used
    // as annotation bounds
    List<PdfQuadrilateral> annotationBounds = new List<PdfQuadrilateral>();
    PdfQuadrilateral quadrilateral = new PdfQuadrilateral(p1, p2, p3, p4);
    annotationBounds.Add(quadrilateral);

    // Add a text markup annotation
    PdfPageFacade pageFacade = pdfViewer1.GetDocumentFacade().Pages[pageNumber-1];
    pageFacade.AddTextMarkupAnnotation(annotationBounds, PdfTextMarkupAnnotationType.Highlight);
}

Public Sub New()
  InitializeComponent()
  pdfViewer1.LoadDocument(filePath)
  AddHandler pdfViewer1.MouseDown, AddressOf pdfViewer1_MouseDown
  AddHandler pdfViewer1.MouseUp, AddressOf pdfViewer1_MouseUp
End Sub

Private startSelectionPosition As PdfDocumentPosition
Private Sub pdfViewer1_MouseDown(ByVal sender As Object, ByVal e As MouseEventArgs)
  ' Get the start position of the selection
  startSelectionPosition = pdfViewer1.GetDocumentPosition(e.Location, True)
  If startSelectionPosition Is Nothing Then
    Return
  End If
End Sub

Private Sub pdfViewer1_MouseUp(ByVal sender As Object, ByVal e As MouseEventArgs)
  ' Retrieve the end position of the selection
  Dim endSelectionPosition As PdfDocumentPosition = pdfViewer1.GetDocumentPosition(e.Location)
  If endSelectionPosition Is Nothing OrElse startSelectionPosition Is Nothing Then
    Return
  End If
  If endSelectionPosition.Point.Equals(startSelectionPosition.Point) Then
    Return
  End If

  ' Define the points to build a quadrilateral
  Dim p1 As PdfPoint = startSelectionPosition.Point
  Dim p4 As PdfPoint = endSelectionPosition.Point
  Dim p2 As New PdfPoint(p4.X, p1.Y)
  Dim p3 As New PdfPoint(p1.X, p4.Y)
  Dim pageNumber As Integer = startSelectionPosition.PageNumber

  ' Create a list of quadrilaterals used
  ' as annotation bounds
  Dim annotationBounds As New List(Of PdfQuadrilateral)()
  Dim quadrilateral As New PdfQuadrilateral(p1, p2, p3, p4)
  annotationBounds.Add(quadrilateral)

  ' Add a text markup annotation
  Dim pageFacade As PdfPageFacade = pdfViewer1.GetDocumentFacade().Pages(pageNumber-1)
  pageFacade.AddTextMarkupAnnotation(annotationBounds, PdfTextMarkupAnnotationType.Highlight)
End Sub