ITextSharp insert text to an existing pdf

In addition to the excellent answers above, the following shows how to add text to each page of a multi-page document:

 using (var reader = new PdfReader(@"C:\Input.pdf"))
 {
    using (var fileStream = new FileStream(@"C:\Output.pdf", FileMode.Create, FileAccess.Write))
    {
       var document = new Document(reader.GetPageSizeWithRotation(1));
       var writer = PdfWriter.GetInstance(document, fileStream);

       document.Open();

       for (var i = 1; i <= reader.NumberOfPages; i++)
       {
          document.NewPage();

          var baseFont = BaseFont.CreateFont(BaseFont.HELVETICA_BOLD, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
          var importedPage = writer.GetImportedPage(reader, i);

          var contentByte = writer.DirectContent;
          contentByte.BeginText();
          contentByte.SetFontAndSize(baseFont, 12);

          var multiLineString = "Hello,\r\nWorld!".Split('\n');

          foreach (var line in multiLineString)
          {
             contentByte.ShowTextAligned(PdfContentByte.ALIGN_LEFT, line, 200, 200, 0);
          }

          contentByte.EndText();
          contentByte.AddTemplate(importedPage, 0, 0);
       }

       document.Close();
       writer.Close();
    }
 }

Here is a method To print over images: taken from here. Use a different layer for your text you're putting over the images, and also make sure to use the GetOverContent() method.

            string oldFile = "FileWithImages.pdf";
            string watermarkedFile = "Layers.pdf";
            // Creating watermark on a separate layer
            // Creating iTextSharp.text.pdf.PdfReader object to read the Existing PDF Document
            PdfReader reader1 = new PdfReader(oldFile);
            using (FileStream fs = new FileStream(watermarkedFile, FileMode.Create, FileAccess.Write, FileShare.None))
            // Creating iTextSharp.text.pdf.PdfStamper object to write Data from iTextSharp.text.pdf.PdfReader object to FileStream object
            using (PdfStamper stamper = new PdfStamper(reader1, fs))
            {
                // Getting total number of pages of the Existing Document
                int pageCount = reader1.NumberOfPages;

                // Create New Layer for Watermark
                PdfLayer layer = new PdfLayer("Layer", stamper.Writer);
                // Loop through each Page
                for (int i = 1; i <= pageCount; i++)
                {
                    // Getting the Page Size
                    Rectangle rect = reader1.GetPageSize(i);

                    // Get the ContentByte object
                    PdfContentByte cb = stamper.GetOverContent(i);

                    // Tell the cb that the next commands should be "bound" to this new layer
                    cb.BeginLayer(layer);
                    
                    BaseFont bf = BaseFont.CreateFont(BaseFont.HELVETICA, BaseFont.CP1252, BaseFont.NOT_EMBEDDED);
                    cb.SetColorFill(BaseColor.RED);
                    cb.SetFontAndSize(bf, 100);

                    cb.BeginText();
                    cb.ShowTextAligned(PdfContentByte.ALIGN_CENTER, "Some random blablablabla...", rect.Width / 2, rect.Height / 2, - 90);
                    cb.EndText();

                    // Close the layer
                    cb.EndLayer();
                }
            }