Using iText (existing PDF in bytes)
public static byte[] mergePDF(List<byte[]> pdfFilesAsByteArray) throws DocumentException, IOException {
ByteArrayOutputStream outStream = new ByteArrayOutputStream();
Document document = null;
PdfCopy writer = null;
for (byte[] pdfByteArray : pdfFilesAsByteArray) {
try {
PdfReader reader = new PdfReader(pdfByteArray);
int numberOfPages = reader.getNumberOfPages();
if (document == null) {
document = new Document(reader.getPageSizeWithRotation(1));
writer = new PdfCopy(document, outStream); // new
document.open();
}
PdfImportedPage page;
for (int i = 0; i < numberOfPages;) {
++i;
page = writer.getImportedPage(reader, i);
writer.addPage(page);
}
}
catch (Exception e) {
e.printStackTrace();
}
}
document.close();
outStream.close();
return outStream.toByteArray();
}