iText has more than one way of doing this. The PdfStamper
class is one option. But I find the easiest method is to create a new PDF document then import individual pages from the existing document into the new PDF.
// Create output PDF
Document document = new Document(PageSize.A4);
PdfWriter writer = PdfWriter.getInstance(document, outputStream);
document.open();
PdfContentByte cb = writer.getDirectContent();
// Load existing PDF
PdfReader reader = new PdfReader(templateInputStream);
PdfImportedPage page = writer.getImportedPage(reader, 1);
// Copy first page of existing PDF into output PDF
document.newPage();
cb.addTemplate(page, 0, 0);
// Add your new data / text here
// for example...
document.add(new Paragraph("my timestamp"));
document.close();
This will read in a PDF from templateInputStream
and write it out to outputStream
. These might be file streams or memory streams or whatever suits your application.