If you share a document with a link, the link is not permanent.
Based on the default setting, the link will live for 60 days
You can eventually reduce the time.
To reduce the time:
See the process on how to share file.
Click on the following link
Change the date with the date picker
Click on "Apply"
It is not possible to increase the date of the permalink
If you want to provide a permanent access, grant access based on user. User need to exist on our SharePoint online or must be an EORTC colleague.