How can I add custom sizes for PDF thumbnails generation?

We have the fallback_intermediate_image_sizes filter within wp_generate_attachment_metadata():

 **
 * Filters the image sizes generated for non-image mime types.
 *
 * @since 4.7.0
 *
 * @param string[] $fallback_sizes An array of image size names.
 * @param array    $metadata       Current attachment metadata.
 */
$fallback_sizes = apply_filters( 'fallback_intermediate_image_sizes', $fallback_sizes, $metadata );

where the default sizes are:

$fallback_sizes = array(
    'thumbnail',
    'medium',
    'large',
);

See core ticket #39231.