gistfile1.cs

string fileRead = @"c:\fileRead.pdf"
string fileWrite = @"c:\fileWrite.pdf"

using (var reader = new PdfReader(fileRead))
{
    using (var stamper = new PdfStamper(reader, new FileStream(fileWrite, FileMode.Create)))
    {
        var info = reader.Info;
        info["Author"] = "Bruno Lowagie";
        info["Title"] = "Hello World stamped";

        stamper.MoreInfo = info;

        using (var ms = new MemoryStream())
        {
            var xmp = new XmpWriter(ms, info);
            stamper.XmpMetadata = ms.ToArray();
            xmp.Close();
        }
    }
}