For those who encounter my problem, I have the solution:

For your model to work and therefore be taken into account, you must add a dependency, here 'sale_pdf_quote_builder' in the __manifest__.py. This allows your model to be loaded after the dependency.

If you want to add fields for filling PDF, add your fields with the field name (Dynamic text in PDF), regarding the module:

# ir_actions_report.py

from odoo import models, api
class IrActionsReport(models.Model):
	​_inherit = 'ir.actions.report'

	​@api.model
	​def _get_form_fields_mapping(self, order, doc_line_id_mapping=None):
	​	​form_fields_mapping = super()._get_form_fields_mapping(order, doc_line_id_mapping=doc_line_id_mapping)
	​	​form_fields_mapping.update({
	​	​	​'signed_on': order.signed_on,
	​	​	​'signed_by': order.signed_by,})
	​	​return form_fields_mapping