Hello yasmina,

You can use below code for get url of the pdf report

base_url = self.env['ir.config_parameter'].get_param('web.base.url')
local_url = self.env['ir.attachment'].browse(new_created_attachment_pdf_report).mapped('local_url')
if base_url and local_url:
URL = base_url + '/'+ local_url

URL is final downloadable url for pdf type attachment created

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari