In order to display a waiting bar or another indicator, while RadPdfViewer is rendering a heavy first page or the end user scrolls pages quickly, it can be used the following code snippet: 

void radPdfViewer1_PageRenderCompleted(object sender, EventArgs e)
{
    if (this.radPdfViewer1.InvokeRequired)
    {
        this.radPdfViewer1.BeginInvoke((MethodInvoker)delegate
        {
            this.radWaitingBar1.StopWaiting();                   
            this.radWaitingBar1.Visible = false;
        }); 
    }
}

void radPdfViewer1_PageRenderStarted(object sender, EventArgs e)
{
    if (this.radPdfViewer1.InvokeRequired)
    {
        this.radPdfViewer1.BeginInvoke((MethodInvoker)delegate
        {
            this.radWaitingBar1.Visible = true;
            this.radWaitingBar1.StartWaiting();
        });
    }
}