By default, when you display a print preview or print a document, the document appears in portrait orientation with one inch margins. To change that, set the PrintDocument's DefaultPageSettings properties before displaying the preview as in the following code.

// Display the print preview.
private void btnGo_Click(object sender, EventArgs e)
{
    pdocTriangle.DefaultPageSettings.Margins =
        new System.Drawing.Printing.Margins(50, 50, 50, 50);
    pdocTriangle.DefaultPageSettings.Landscape = true;
    ppdTriangle.ShowDialog();
}

This code sets the page's margins to half inch (measured in hundredths of inches) and makes it appear in landscape orientation. It then displays the print preview dialog.