List GUIDs of all attachments in WordPress Media Library, limited by mime-type, e.g, PDFs only.

wp-list-media-limit-mime-type.php

<?php
/* List GUIDs of all files of specified mime-type in Media Library. */
$query_images_args = array(
    'post_type'      => 'attachment',
    'post_status'    => 'inherit',
    'post_mime_type' => 'application/pdf', // Only PDFs.
    'posts_per_page' => -1,
);
$query_images = new WP_Query( $query_images_args );
print_r( wp_list_pluck( $query_images->posts, 'guid' ) );
?>