Why an user with "Everything" permission cannot download files?
I created a document with type File and attached a file to it, so I created a user called 'demo' and added “Everything” permission on the “root” folder to this user. Now the 'demo' user has access to all documents inside root folder as expected, but he can't download files even with “Everything” permission. Only administrator users can download files on nuxeo?
You need READ permission to download a document so there's must be something wrong on your side. Try making an XML export / JSON export with the Browser Extension to see the ACL.
I cannot reproduce your issue.