Sort WordPress media library by pdf file type.

functions.php

//sort media library by pdf file type - http://wordpress.org/support/topic/filter-media-library-by-file-type
function modify_post_mime_types($post_mime_types) {
    $post_mime_types['application/pdf'] = array(__('PDF'), __('Manage PDF'), _n_noop('PDF <span class="count">(%s)</span>', 'PDF <span class="count">(%s)</span>'));
    return $post_mime_types;
}
add_filter('post_mime_types', 'modify_post_mime_types');