Check my custom module https://apps.odoo.com/apps/modules/18.0/hh_elearning_custom_certificate

1. Customizing Certificates

• Odoo offers a default certificate template, but you can customize it.
• You’ll need to customize the template by going to Settings > Technical > Reports > Reports.
• Find the "Certificate" report and modify the QWeb template or download the template, edit the XML to fit your custom design, and re-upload it.

2. Automatically Email Certificates

To automatically send certificates to students via email:

• Go to Settings > Automation > Automated Actions.
• Create a new action with:
  • Trigger: When a student passes a course or quiz.
  • Action: Send an email.
• Attach the custom certificate in PDF format to the email template.