CodeIgniter 3 With IIS 7.5 - Best Code for File Downloads

I am relatively new to CI. I have a working site with basic login/logout code in place.  I store the user's userid in a session. My next task is to provide a download link for each file listed in the database (files are stored outside of the webroot). I am not a newbie to PHP but not an expert either. I am looking for a solution that is easy to understand and contains the basic code.  

Here are the parameters for downloading files on my site. 

• IIS 7.5 Web server
  
• PHP 7
  
• CodeIgniter 3
  
• Files stored outside of webroot
  
• Only logged in users can download files
  
• No file path information will be stored in the database
  
• Content types will be images, MS Office docs, PDFs, mp3, mp4
  

Any recommendations are greatly appreciated.

I've seen this solution, but it is only for downloading files residing within the webroot: http://www.technicalkeeda.com/codeignite...ad-example