existing_pdf = PdfFileReader(file('original.pdf', 'rb'))
output = PdfFileWriter()
page = existing_pdf.getPage(0)
page.mergePage(new_pdf.getPage(0))
output.addPage(page)
outputStream = file('destination.pdf', 'wb')
output.write(outputStream)
outputStream.close()
Add text to Existing PDF using Python