Add post type support for the post excerpt

post-type-support.php

<?php
/**
 * Add support for the excerpt on pages.
 */
function hd_add_custom_post_type_excerpt_support() {

    // add post type support for pages.
    add_post_type_support( 'page', 'excerpt' );

    // add post type support for case studies.
    add_post_type_support( 'pmd_case_study', 'excerpt' );

}

add_action( 'init', 'hd_add_custom_post_type_excerpt_support' );