Workaround: until the feature be implemented one can print each page individually respecting its orientation whether it is portrait or landscape
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
 
        this.radPdfViewer1.LoadDocument(@"..\..\sample.pdf");
    }
 
    private void radButton1_Click(object sender, EventArgs e)
    {
        RadPrintDocument doc = new RadPrintDocument();
        doc.AssociatedObject = this.radPdfViewer1;
 
        for (int i = 0; i < this.radPdfViewer1.Document.Pages.Count; i++)
        {
            RadFixedPage page = this.radPdfViewer1.Document.Pages[i];
 
            doc.CurrentPage = i;
            doc.PrinterSettings.PrintRange = PrintRange.CurrentPage;
            doc.Landscape = page.ActualWidth > page.ActualHeight;
 
            doc.Print();
        }
    }
}