// Create a new document.
using (DocX document = DocX.Create(@"C:\Users\Cathal\Desktop\Hello.docx"))
{
    // Set this documents orientation to landscape and save.
    document.PageLayout.Orientation = Orientation.Landscape;
    document.SaveAs(@"Landscape.docx");

    // Set this documents orientation to portrait and save.
    document.PageLayout.Orientation = Orientation.Portrait;
    document.SaveAs(@"Portrait.docx");
}