このコードを試してください。
File firstPdfFile = new File("firstPdf.pdf");
File secondPdfFile = new File("firstPdf.pdf");
PDFMergerUtility merger = new PDFMergerUtility();
merger.addSource(firstPdfFile);
merger.addSource(secondPdfFile);
String pdfPath = "PdfFile.pdf";
OutputStream bout2 = new BufferedOutputStream(new FileOutputStream(pdfPath));
merger.setDestinationStream(bout2);
merger.mergeDocuments();
PDDocument doc = null;
try {
URL file = new URL("file:///PdfFile.pdf");
doc = PDDocument.load(file);
List<?> allPages = doc.getDocumentCatalog().getAllPages();
PDFont font = PDType1Font.HELVETICA_BOLD;
float fontSize = 36.0f;
for (int i = 0; i < allPages.size(); i++) {
PDPage page = (PDPage) allPages.get(i);
PDPageContentStream footercontentStream = new PDPageContentStream(doc, page, true, true);
footercontentStream.beginText();
footercontentStream.setFont(font, fontSize);
footercontentStream.moveTextPositionByAmount((PDPage.PAGE_SIZE_A4.getUpperRightX() / 2), (PDPage.PAGE_SIZE_A4.getLowerLeftY()));
footercontentStream.drawString(String.valueOf(i + 1));
footercontentStream.endText();
footercontentStream.close();
}
doc.save("PdfFile.pdf");
} finally {
if (doc != null) {
doc.close();
}
}