it turns out that iText can do the same thing.Here is the code for that :
PdfReader reader = new PdfReader("Hello World.pdf");
Document document = new Document(PageSize.A4);
PdfWriter writer = PdfWriter.getInstance(document,
new FileOutputStream("RESULT.PDF"));
document.open();
PdfContentByte canvas = writer.getDirectContent();
PdfImportedPage page;
for (int i = 3; i <=6; i++) {
page = writer.getImportedPage(reader, i);
canvas.addTemplate(page, 1f, 0, 0, 1, 0, i*30-250);
}
document.close();