PDF4me Add Watermark tool comes with a lot of handy customizations. You can choose to add Text or Image watermarks. Customize a lot of properties like the size, rotation, position, or even the layer where the Watermark is added.
Using the Watermark tool, you can either set the stamp/ watermark to be placed on the top layer of the PDF or towards the background. You can also set the transparency of the Watermark as well.
Sometimes, there are chances that the PDF might have a lot of opaque layers in the content and if you place the Watermark in the background in such cases, it might not reflect in the final document. In such circumstances, you can choose to set the stamp to the foreground and this would mostly resolve the issue of the Watermark from missing in the final PDF.