Workaround: print the document page by page:
private void radButton1_Click(object sender, EventArgs e)
{
    RadPrintDocument d = new RadPrintDocument();
    d.PrinterSettings.PrintRange = PrintRange.CurrentPage;
    d.AssociatedObject = this.radPdfViewer1;
    for (int i = 0; i < this.radPdfViewer1.Document.Pages.Count; i++)
    {
        d.CurrentPage = i;
        d.Print();
    }
}