register_taxonomy_for_object_type('category','document');

'taxonomies' => array('category',),

$labels = array(
        'name' => _x('Document', 'post type general name', 'creative'),
        'singular_name' => _x('Document', 'post type singular name', 'creative'),
        'add_new' => _x('Add New', 'post type new', 'creative'),
        'add_new_item' => __('Add New Document', 'creative'),
        'edit_item' => __('Edit Document', 'creative'),
        'new_item' => __('New Document', 'creative'),
        'view_item' => __('View Document', 'creative'),
        'search_items' => __('Search Documents', 'creative'),
        'not_found' =>  __('No Documents found', 'creative'),
        'not_found_in_trash' => __('No Documents found in Trash', 'creative'),
        'parent_item_colon' => '',
        'menu_name' => 'Documents'
        );
$args = array(
        'labels' => $labels,
        'taxonomies' => array('category',),
        'public' => true,
        'publicly_queryable' => true,
        'show_ui' => true,
        'show_in_menu' => true,
        'query_var' => true,
        'rewrite' => true,
        'capability_type' => 'post',
        'has_archive' => false,
        'hierarchical' => true,
        'menu_position' => 20,
        'menu_icon' => 'dashicons-media-default',
        'supports' => array( 'title', 'editor', 'thumbnail' )
    );
register_post_type( 'document', $args);