try as below
AddPageNumber("bill3.pdf","sunil1.pdf");
void AddPageNumber(string fileIn, string fileOut)
{
byte[] bytes = File.ReadAllBytes(fileIn);
Font blackFont = FontFactory.GetFont("Arial", 12, Font.NORMAL, BaseColor.BLACK);
using (MemoryStream stream = new MemoryStream())
{
PdfReader reader = new PdfReader(bytes);
using (PdfStamper stamper = new PdfStamper(reader, stream))
{
int pages = reader.NumberOfPages;
for (int i = 1; i <= pages; i++)
{
ColumnText.ShowTextAligned(stamper.GetUnderContent(i), Element.ALIGN_RIGHT, new Phrase(i.ToString(), blackFont), 568f, 15f, 0);
}
}
bytes = stream.ToArray();
}
File.WriteAllBytes(fileOut, bytes);
}