Regenerate Thumbnails

You are correct, thumbnails is generated by hooking into the wp_generate_attachment_metadata hook, which triggers when you upload a new file.

Unfortunately, the regenerate thumbnails plugin only loops through images, so it will not trigger thumbnail generation for PDFs. It would be useful to support the regenerate thumbnails plugin. I will check with the author to see if we can figure something out. However, I reckon that will take some time.

For now, you can do it manually with the following code snippet:

add_action('admin_init', function () {

    if (!isset($_GET['regenerate_pdf_thumbnails'])) {
        return;
    }

    $chunk_size = empty($_GET['regenerate_pdf_thumbnails_chunk_size']) ? -1
        : $_GET['regenerate_pdf_thumbnails_chunk_size'];

    // Get all attachments
    $posts = get_posts(array(
        'post_type' => 'attachment',
        'post_status' => 'any',
        'posts_per_page' => $chunk_size,
        'fields' => 'ids',
        'paged' => (int) $_GET['regenerate_pdf_thumbnails'],
        'meta_key' => 'PdfThumbnailsPlugin',
        'meta_compare' => 'NOT EXISTS'
    ));

    // Regenerate metadata (incl. thumbnails and pdf thumnails)
    foreach ($posts as $id) {
        $file = get_attached_file($id);
        $meta = wp_generate_attachment_metadata($id, $file);
        update_post_meta($id, $meta);
    }
});

With that you can use http://YOURSITE?regenerate_pdf_thumbnails to trigger the generation. To avoid PHP running for too long, and cancelling the operation, you can use:

Regenerate attachments 1-5:
http://YOURSITE/wp-admin/upload.php?regenerate_pdf_thumbnails=1&regenerate_pdf_thumbnails_chunk_size=5

Regenerate attachments 6-10:
http://YOURSITE/wp-admin/upload.php?regenerate_pdf_thumbnails=1&regenerate_pdf_thumbnails_chunk_size=5

And so on.

Good look with your imports, marking this as resolved 🙂