In Portal 3.0 and Workspaces, we do not have a way to make any fields read-only. Any fields that are added to your template will be editable by your users.
In Portal 2.0, there is no setting that can make your fields read-only, but we do have a Data Hook® example that will grab the information submitted when the file was uploaded for a column and overwrite any edits your users make to that field to ensure the original data persists in that field. Please note: they will still be able to edit the field, but this Data Hook will ensure the original data they submitted to that field overwrites any edits to that field. To see how to implement this Data Hook, click here.