downloads_controller.rb

class DownloadsController < ApplicationController
  def show
    if @current_user
      send_file "#{Rails.root}/downloads/test.pdf"
    else
      render :file => "#{Rails.root}/public/404.html", :status => 404, :layout => false
    end
  end
end

nginx.conf

# nginx X-Accel-Redirect configuration for Rails and Unicorn 

upstream unicorns {
    server unix:/srv/APPLICATION/tmp/sockets/unicorn.sock;
    # If you're not using unicorn with UNIX domain sockets, you'd do something like this:
    # server 127.0.0.01:8080;
}

server {
    listen       80;
    server_name  example.org;

    root /srv/APPLICATION/public;
    
    location ~ /downloads/(.*) {
      internal;
      alias /srv/APPLICATION/downloads/$1;
    }
    
    location / {
      proxy_redirect    off;
      
      proxy_set_header  Host              $http_host;
      proxy_set_header  X-Real-IP         $remote_addr;
      proxy_set_header  X-Forwarded-For   $proxy_add_x_forwarded_for;

      proxy_set_header  X-Sendfile-Type   X-Accel-Redirect;
      proxy_set_header  X-Accel-Mapping   /downloads/=/srv/APPLICATION/downloads/;
      
      proxy_pass http://unicorns;
    }
}

production.rb

# Specifies the header that your server uses for sending files
#config.action_dispatch.x_sendfile_header = "X-Sendfile"

# For nginx:
config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect'

# If you have no front-end server that supports something like X-Sendfile,
# just comment this out and Rails will serve the files