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A stamp annotation displays graphics, images, or text on a page to resemble a stamped mark. When opened, the annotation displays a pop-up window containing the associated note.

Use the StampAnnotation class to add stamp annotations to a PDF document.
const doc = new PdfDocument();
const page = doc.newPage();
// Create an instance of StampAnnotation class and set its relevant properties
const stamp = new StampAnnotation();
stamp.contents = "This is a sample stamp";
stamp.userName = "Jamie Smith";
stamp.color = "SkyBlue";
stamp.icon = "Confidential";
stamp.creationDate = Date.now();
stamp.rect = { x: 100.5, y: 110.5, width: 72, height: 72 };
page.annotations.add(stamp); //Add the stamp annotation
const docData = doc.savePdf();
Util.saveFile("StampAnnotation.pdf", docData);