How to position iTextSharp paragraph to bottom of the page?

You can create Footer class and use it.

Create a class that inherited by PdfPageEventHelper.

Create table in this class and write footer content.

public partial class Footer : PdfPageEventHelper
        {
            public override void OnEndPage(PdfWriter writer, Document doc)
            {
                Paragraph footer = new Paragraph("THANK YOU", FontFactory.GetFont(FontFactory.TIMES, 10, iTextSharp.text.Font.NORMAL));
                footer.Alignment = Element.ALIGN_RIGHT;
                PdfPTable footerTbl = new PdfPTable(1);
                footerTbl.TotalWidth = 300;
                footerTbl.HorizontalAlignment = Element.ALIGN_CENTER;
                PdfPCell cell = new PdfPCell(footer);
                cell.Border = 0;
                cell.PaddingLeft = 10;
                footerTbl.AddCell(cell);
                footerTbl.WriteSelectedRows(0, -1, 415, 30, writer.DirectContent);
            }
        }

After this

Document document = new Document(PageSize.A4, 50, 50, 25, 25);
var output = new FileStream(Server.MapPath("Demo.pdf"), FileMode.Create);
PdfWriter writer = PdfWriter.GetInstance(document, output);
// Open the Document for writing
document.Open();
//using footer class
writer.PageEvent = new Footer();.
Paragraph welcomeParagraph = new Paragraph("Hello, World!");
document.Add(welcomeParagraph);
document.Close();

Original article

And another way - You can simply add code below

Paragraph copyright = new Paragraph("© 2020 AO XXX. All rights reserved.", calibri8Black);
PdfPTable footerTbl = new PdfPTable(1);
footerTbl.TotalWidth = 300;
PdfPCell cell = new PdfPCell(copyright);
cell.Border = 0;
footerTbl.AddCell(cell);
footerTbl.WriteSelectedRows(0, -1, 30, 30, writer.DirectContent);