<?php
    $terms_array = array(
      'taxonomy' => 'post_tag',
        );
    $services_terms = get_terms($terms_array);
    foreach($services_terms as $service): ?>
    <h4><?php echo $service->name; ?></h4>
    <?php
    $post_args = array(
            'post_type' => 'attachment',
            'post_mime_type' => ['application/pdf'],
            'numberposts' => -1,
            'post_status' => null,
            'post_parent' => null,
            'tax_query' => array(
                    array(
                        'taxonomy' => 'post_tag',
                        'field' => 'slug',
                        'terms' => 'sub', // ['sub', 'ins'] does not work
                    )
                )
            );
    $myposts = get_posts($post_args); ?>
    <ul>
    <?php foreach ( $myposts as $post ) : setup_postdata( $post ); ?>
      <li>
        <a href="<?php the_permalink(); ?>"><?php the_title(); ?></a>
      </li>
    <?php endforeach; // Term Post foreach ?>
    </ul>
    <?php wp_reset_postdata(); ?>

    <?php endforeach; // End Term foreach; ?>