The following piece of code allows you to draw red border around the specified page of your document. No hard-coding except page number.
PdfPage page = pdfDocument.getPage(1);
Rectangle pageRect = new Rectangle(page.getTrimBox());
new PdfCanvas(page).setStrokeColor(ColorConstants.RED).setLineWidth(5).rectangle(pageRect).stroke();
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solved IText 7 – Adding border for the page