watermark.py

from PyPDF2 import PdfFileWriter, PdfFileReader

def create_watermark(input, output, watermark):
    watermark_obj = PdfFileReader(watermark)
    watermark_page = watermark_obj.getPage(0)

    pdf_reader = PdfFileReader(input_pdf)
    pdf_writer = PdfFileWriter()

    # Watermark all the pages
    for page in range(pdf_reader.getNumPages()):
        page = pdf_reader.getPage(page)
        page.mergePage(watermark_page)
        pdf_writer.addPage(page)
    
    #output the pdf
    with open(output, 'wb') as out:
        pdf_writer.write(out)



input='sample.pdf', 
output='watermark_pdf.pdf',
watermark='watermark.pdf'

watermark_Pdf(input,output,watermark)