I created an UnequalPages example that shows how it works:
Document document = new Document();
PdfWriter.getInstance(document, new FileOutputStream(dest));
Rectangle one = new Rectangle(70,140);
Rectangle two = new Rectangle(700,400);
document.setPageSize(one);
document.setMargins(2, 2, 2, 2);
document.open();
Paragraph p = new Paragraph("Hi");
document.add(p);
document.setPageSize(two);
document.setMargins(20, 20, 20, 20);
document.newPage();
document.add(p);
document.close();
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