is_page() is not working on the admin area so I needed to use the following code
function this_screen() {
$current_screen = get_current_screen();
if( $current_screen ->id === "page" ) {
include_once 'page_admin.php';
}
}
add_action( 'current_screen', 'this_screen' );
You can replace “page” to any custom post type name