Here is a method that uses stamper and absolute coordinates showed in the different PDF clients (Adobe, FoxIt and etc. )
public static void AddTextToPdf(string inputPdfPath, string outputPdfPath, string textToAdd, System.Drawing.Point point)
{
//variables
string pathin = inputPdfPath;
string pathout = outputPdfPath;
//create PdfReader object to read from the existing document
using (PdfReader reader = new PdfReader(pathin))
//create PdfStamper object to write to get the pages from reader
using (PdfStamper stamper = new PdfStamper(reader, new FileStream(pathout, FileMode.Create)))
{
//select two pages from the original document
reader.SelectPages("1-2");
//gettins the page size in order to substract from the iTextSharp coordinates
var pageSize = reader.GetPageSize(1);
// PdfContentByte from stamper to add content to the pages over the original content
PdfContentByte pbover = stamper.GetOverContent(1);
//add content to the page using ColumnText
Font font = new Font();
font.Size = 45;
//setting up the X and Y coordinates of the document
int x = point.X;
int y = point.Y;
y = (int) (pageSize.Height - y);
ColumnText.ShowTextAligned(pbover, Element.ALIGN_CENTER, new Phrase(textToAdd, font), x, y, 0);
}
}