public partial class Form1 : RibbonForm
{
    Cursor currentCursor = null;

    //This property checks whether the cursor is located within the document's borders
    bool CursorIsOverPage { get { return pdfViewer.IsDocumentOpened && pdfViewer.GetDocumentPosition(pdfViewer.PointToClient(MousePosition), false) != null; } }

    public Form1()
    {
        InitializeComponent();
        pdfViewer.CursorMode = PdfCursorMode.Custom;
        pdfViewer.MouseMove += PdfViewer_MouseMove;
        currentCursor = pdfViewer.Cursor;

    }

    private void PdfViewer_MouseMove(object sender, MouseEventArgs e)
    {
        //Show the No cursor when it hovers over the document
        pdfViewer.Cursor = currentCursor != null && CursorIsOverPage ? Cursors.No : currentCursor;
    }
}

Partial Public Class Form1
    Inherits RibbonForm

    Private currentCursor As Cursor = Nothing

    'This property checks whether the cursor is located within the document's borders
    Private ReadOnly Property CursorIsOverPage() As Boolean
        Get
            Return pdfViewer.IsDocumentOpened AndAlso pdfViewer.GetDocumentPosition(pdfViewer.PointToClient(MousePosition), False) IsNot Nothing
        End Get
    End Property

    Public Sub New()
        InitializeComponent()
        pdfViewer.CursorMode = PdfCursorMode.Custom
        AddHandler pdfViewer.MouseMove, AddressOf PdfViewer_MouseMove
        currentCursor = pdfViewer.Cursor

    End Sub

    Private Sub PdfViewer_MouseMove(ByVal sender As Object, ByVal e As MouseEventArgs)
        'Show the No cursor when it hovers over the document
        pdfViewer.Cursor = If(currentCursor IsNot Nothing AndAlso CursorIsOverPage, Cursors.No, currentCursor)
    End Sub
End Class