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<?php  //<~ don't add me in

function wpb_remove_posts($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_search) {
      $query->set('post_type', 'page');
    }
  }
}

add_action('pre_get_posts','wpb_remove_posts');

<?php  //<~ don't add me in

function wpb_remove_pages($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_search) {
      $query->set('post_type', 'post');
    }
  }
}

add_action('pre_get_posts','wpb_remove_pages');

<?php  //<~ don't add me in

function wpb_add_cpt_search($query) {
  if ( !is_admin() && $query->is_main_query() ) {
    if ($query->is_search) {
      $query->set('post_type', array('page', 'post', 'custom-post-type-name'));
    }
  }
}

add_action('pre_get_posts','wpb_add_cpt_search');