write the following in your code

 <?php
 add_action( 'wp_enqueue_scripts', 'theme_enqueue_styles' );
 function theme_enqueue_styles() {
 wp_enqueue_style( 'parent-style', get_template_directory_uri() . '/style.css' );
 }

 function my_scripts_enqueue() {
 wp_register_script( 'bootstrap-js', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js', array('jquery'), NULL, true );
 wp_register_style( 'bootstrap-css', 'https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css', false, NULL, 'all' );

 wp_enqueue_script( 'bootstrap-js' );
 wp_enqueue_style( 'bootstrap-css' );
 }
 add_action( 'wp_enqueue_scripts', 'my_scripts_enqueue' );
 ?>

it will download your file