how to send a file to a specific user? I have a content type which has an entity reference pointing to users. I need to have a permission policy that says: only users who are defined in the content (as entity reference) can view it or download the file attached to it. anybody else should be faced with a message which saying: It's not your business!!

help please
You can try this module: https://www.drupal.org/project/nodeaccess_userreference. I use it too and it works very well.

As the module's page says, it "Gives content access permissions to users for content that references the users with User reference or Entity reference."

With this module you can define permissions just for the entire entity, you can't define them to the single fields.