How to remove watermark created by spire.pdf?

gistfile1.java

 PDDocument document = PDDocument.load(new File("files/test.pdf"));
 for (PDPage page : document.getPages()) {
     PDResources resources = page.getResources();
     resources.getCOSObject().removeItem(COSName.PATTERN);
 }
 document.save("files/1.pdf");

Spire.pdf usually use a PDAbstractPattern to create a watermark, so If we delete the pattern, the watermark won't be rendered.