Install the Python library PyPDF2, which provides functionality for manipulating PDFs. You can install it using pip:
pip install PyPDF2

1. Create a Python function that merges the PDFs and adds the combined header. Here's an example:

import PyPDF2

def merge_pdfs_with_combined_header(pdf_paths, header_text, output_path):
    merger = PyPDF2.PdfFileMerger()

    # Read each PDF and add it to the merger object
    for pdf_path in pdf_paths:
        merger.append(pdf_path)

    # Add the combined header to each page of the merged PDF
    for page_num in range(merger.getNumPages()):
        page = merger.getPage(page_num)
        page.mergePage(PyPDF2.pdf.PageObject.createBlankPage(page))
        header = PyPDF2.pdf.PageObject.createTextObject(header_text)
        page.mergeTranslatedPage(header, 0, page.mediaBox.getHeight())

    # Save the merged PDF with the combined header
    merger.write(output_path)
    merger.close()

Hi,

If you are familiar with code , you can try use the following

attachment_report_id = self.env['ir.attachment'].sudo().create(
    report_values)
# self.env.cr.commit()
merged_attach_ids = attachment_report_id._full_path(
    attachment_report_id.store_fname)

pdfWriter = PyPDF2.PdfFileWriter()
for list in attachment_list:
    pdf1File = open(list, 'rb')
    pdf1Reader = PyPDF2.PdfFileReader(pdf1File)
    #
    for pageNum in range(pdf1Reader.numPages):
        pageObj = pdf1Reader.getPage(pageNum)
        pdfWriter.addPage(pageObj)
#
for page in range(new_pdf.numPages):
    pageObj1 = new_pdf.getPage(page)
    pdfWriter.addPage(pageObj1)

pdfOutputFile = open(merged_attach_ids, 'wb')
pdfWriter.write(pdfOutputFile)
pdfOutputFile.close()
pdf1File.close()

Regards