Workaround: 

public Form1()
{
    InitializeComponent();

    string pdfFilePath = Path.Combine(Application.StartupPath, "testfile1.pdf");
    radPdfViewer1.LoadDocument(pdfFilePath);

    this.radPdfViewer1.PdfViewerElement.VScrollBar.ValueChanged+=VScrollBar_ValueChanged;
}

private void VScrollBar_ValueChanged(object sender, EventArgs e)
{
    this.radPdfViewer1.PdfViewerElement.InvalidatePages();
}