how to send a file to a specific user?
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.