Exemplo de código que consegue escrever informações em um PDF editável.

write_in_template_pdf.py

import os
from pathlib import Path

import pdfrw


TEMPLATE_PATH = '/path/to/invoice_template.pdf'
OUTPUT_PATH = '/path/to/invoice.pdf'

ANNOT_KEY = '/Annots'
ANNOT_FIELD_KEY = '/T'
SUBTYPE_KEY = '/Subtype'
WIDGET_SUBTYPE_KEY = '/Widget'


def write_pdf(input_pdf, output_pdf, data_dict):
    template_pdf = pdfrw.PdfReader(os.path.join(Path.home(), input_pdf))

    annotations = template_pdf.pages[0][ANNOT_KEY]

    for annotation in annotations:
        if annotation[SUBTYPE_KEY] == WIDGET_SUBTYPE_KEY:
            if annotation[ANNOT_FIELD_KEY]:
                key = annotation[ANNOT_FIELD_KEY][1:-1]
                if key in data_dict.keys():
                    update = {
                        'V': data_dict[key],
                    }
                    annotation.update(pdfrw.PdfDict(**update))
        annotation.update(pdfrw.PdfDict(Ff=1))

    template_pdf.Root.AcroForm.update(pdfrw.PdfDict(NeedAppearances=pdfrw.PdfObject('true')))
    pdfrw.PdfWriter().write(os.path.join(Path.home(), output_pdf), template_pdf)

data_dict = {
   'business_email_address': 'contato@dunderlabs.com',
   'business_name_1': 'Dunderlabs',
   'business_name_2': 'Dunderlabs',
   'business_phone_number': '(34) 2222-2222',
   'customer_email': 'mazulo@dunderlabs.com',
   'customer_name': 'Patrick Mazulo',
   'due_date': '30/05/2020',
   'invoice_number': '5786878',
   'item_1_amount': 'R$8000',
   'item_1_price': 'R$200/hr',
   'item_1_quantity': '40 hours',
   'item_1': 'Criação de blog',
   'note_contents': 'Muito obrigado por realizar essa compra!',
   'send_date': '29/05/2020',
   'subtotal': 'R$8000',
   'total': 'R$8000',
}

if __name__ == '__main__':
    write_pdf(TEMPLATE_PATH, OUTPUT_PATH, data_dict)