It is better to use CDN for all static files like css/js/icons/images.
Reason: Main purpose of CDN is deliver content from closest point of presence instead of the origin server. If you use the mechanism of keeping the static files in your server and serving it to the client, then it'll affect your server performance. Example, Think about all your users accessing the bootstrap javascript files from your server! Boom! Your server has to do extra effort of delivering those files to the browser. It's an unnecessary work. Thats where CDN comes and helps.