Hi,

You can attach the pdf to the sale order template like this.

Override this function action_quotation_send()

attachment = self.env.ref(
            'Your_module_name.action_name_report',
            raise_if_not_found=False)
 
   ctx = {
            'default_model': 'sale.order',
            'default_res_id': self.ids[0],
            'default_use_template': bool(template_id),
            'default_template_id': template_id,
            'default_composition_mode': 'comment',
            'mark_so_as_sent': True,
            'default_attachment_ids': attachment.ids,
            'custom_layout': "mail.mail_notification_paynow",
            'proforma': self.env.context.get('proforma', False),
            'force_email': True,
            'model_description': self.with_context(lang=lang).type_name,
        }
        return {
            'type': 'ir.actions.act_window',
            'view_mode': 'form',
            'res_model': 'mail.compose.message',
            'views': [(False, 'form')],
            'view_id': False,
            'target': 'new',
            'context': ctx,
        }  

Hope it helps