c# - How to add a watermark to a PDF file?

Question

I'm using C# and iTextSharp to add a watermark to my PDF files:

Document document = new Document();
PdfReader pdfReader = new PdfReader(strFileLocation);
PdfStamper pdfStamper = new PdfStamper(pdfReader, new FileStream(strFileLocationOut, FileMode.Create, FileAccess.Write, FileShare.None));
iTextSharp.text.Image img = iTextSharp.text.Image.GetInstance(WatermarkLocation);
img.SetAbsolutePosition(100, 300);
PdfContentByte waterMark;
//    
for (int pageIndex = 1; pageIndex <= pdfReader.NumberOfPages; pageIndex++)
{
    waterMark = pdfStamper.GetOverContent(pageIndex);
    waterMark.AddImage(img);
}
//
pdfStamper.FormFlattening = true;
pdfStamper.Close();

It works fine, but my problem is that in some PDF files no watermark is added although the file size increased, any idea?

Answer 1

The fact that the file size increases is a good indication that the watermark is added. The main problem is that you're adding the watermark outside the visible area of the page. See How to position text relative to page using iText?

You need something like this:

Rectangle pagesize = reader.GetCropBox(pageIndex);
if (pagesize == null)
    pagesize = reader.GetMediaBox(pageIndex);
img.SetAbsolutePosition(
    pagesize.GetLeft(),
    pagesize.GetBottom());

That is: if you want to add the image in the lower-left corner of the page. You can add an offset, but make sure the offset in the x direction doesn't exceed the width of the page, and the offset in the y direction doesn't exceed the height of the page.