Sub New()

        ' This call is required by the designer.
        InitializeComponent()

        AddHandler Me.RadPdfViewer1.ContainerElement.MouseDown, AddressOf RadPdfViewer_MouseDown
        AddHandler Me.RadPdfViewer1.ContainerElement.MouseUp, AddressOf RadPdfViewer_MouseUp
    End Sub

    Private Sub RadPdfViewer_MouseUp(sender As Object, e As MouseEventArgs)
        If IsInResizeLocation(e.Location) AndAlso isBookmarksSelected Then
            isBookmarksSelected = False
            Me.RadPdfViewer1.ShowBookmarks()
        End If
    End Sub

    Dim isBookmarksSelected As Boolean = False
    Private Sub RadPdfViewer_MouseDown(sender As Object, e As MouseEventArgs)
        If IsInResizeLocation(e.Location) Then
            If Not Me.RadPdfViewer1.ContainerElement.IsBookmarksHidden Then
                isBookmarksSelected = True
            End If 
        End If 
    End Sub

    Private Function IsInResizeLocation(ByVal location As Point) As Boolean
        If Me.ElementTree Is Nothing Then
            Return False
        End If

        Dim element As RadElement = Me.RadPdfViewer1.ContainerElement.ElementTree.GetElementAtPoint(New Point(location.X, location.Y)) 
        If element Is Nothing Then
            Return False
        End If

        If TypeOf element Is PdfSizeGripElement Then
            Return True
        End If

        Return False
    End Function