How to add an image watermark to a PDF file?

My problem is that in some PDF files no watermark is added although the file size increased. Why is that?

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();

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 my answer to the question “How to position text relative to page using iText?” In Java if you use iText 7 you need something like this (use the same methods in C#):

Rectangle pagesize = pdfDoc.getPage(pageIndex).getCropBox();
if (pagesize == null)
    pagesize = pdfDoc.getPage(pageIndex).getMediaBox();

Image img = new Image(ImageDataFactory.create(imageSrc));
img.setFixedPosition (
        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.

Click How to add an image watermark to a PDF file? if you want to see how to answer this question in iText 5.