Document migraDocDocument = new Document();

migraDocDocument.Styles["Normal"].Font.Name = "Courier New";

var section = migraDocDocument.AddSection();
section.Headers.FirstPage.AddParagraph("First page header");
section.PageSetup.DifferentFirstPageHeaderFooter = true;
section.Headers.Primary.AddParagraph("This is my header");
section.PageSetup.PageFormat = PageFormat.Letter;
section.AddParagraph(
        "Lorem ipsum dolor sit amet"
    );

PdfDocumentRenderer pdfDocumentRenderer = new PdfDocumentRenderer();
pdfDocumentRenderer.Document = migraDocDocument;
pdfDocumentRenderer.RenderDocument();

using (MemoryStream memoryStream = new MemoryStream())
{
    pdfDocumentRenderer.PdfDocument.Save(memoryStream, false);

    // Open the PDF document from the memory stream with the desired PdfDocumentOpenMode
    using (var streamedPdfDocument = PdfReader.Open(memoryStream, PdfDocumentOpenMode.Import))
    {
        foreach (var current in streamedPdfDocument.Pages)
        {
            document.AddPage(current);
        }
    }
}