Cannot add image to scanned Pdf or Pdf converted from Html

Using iText (com.lowagie.itext.2.1.7), I have this code that adds a little image to a Pdf. I works with simple Pdf files, but not when I create the Pdf from Html or when the Pdf is a scanned document.

private final Map<String, Image> images = new HashMap<String, Image>();

images.put("NL" + i, getBlackAndWhiteBarCode("/img/NotLast0" + i + ".png", posX, posY));
images.put("L" + i, getBlackAndWhiteBarCode("/img/Last0" + i + ".png", posX, posY));

public Image getBlackAndWhiteBarCode(String filename, float posX, float posY) throws IOException, DocumentException {
	Image im = Image.getInstance(this.getClass().getResource(filename));
	im.setOriginalType(Image.ORIGINAL_PNG);
	im.setAbsolutePosition(posX, posY);
	im.scalePercent(25);
	return im;
}

...

PdfContentByte under = stamp.getUnderContent(pageOfPDF);
under.addImage(images.get(feuilletCpt < feuilletTot ? "NL" + seq : "L" + seq), true);

Example image : enter image description here
Example Pdf : a simple html converted to Pdf by https://html2pdf.com/

It works with every kind of Pdf files, generated, downloaded, but not when I convert an Html to Pdf or when a document is Scanned to Pdf.

Thank you for your help

You add to the UnderContent:

PdfContentByte under = stamp.getUnderContent(pageOfPDF);
under.addImage(images.get(feuilletCpt < feuilletTot ? "NL" + seq : "L" + seq), true);

Thus, the addition can only be seen where there is no previous content. In case of scanned documents there usually is a page-covering image, so your addition underneath is not visible. Some PDF generators first add a page covering white rectangle. In case of such documents, your addition underneath also is not visible.

Use the Overcontent instead: stamp.getOverContent(pageOfPDF)

Another possible issue: The code you show does not take the coordinate system of the page into account. You may have to add the lower left corner of the page crop box to your coordinates.