Workaround: subscribe the RadPdfViewer control to the ViewerModeChanged event and in the handler, if the viewer mode is set to Pan, call the HideCaret method of the PdfViewerElement
private void radPdfViewer1_ViewerModeChanged(object sender, EventArgs e)
{
    if (this.radPdfViewer1.ViewerMode == Telerik.WinControls.UI.FixedDocumentViewerMode.Pan)
    {
        this.radPdfViewer1.PdfViewerElement.HideCaret();
    }
}