Missing caption under pdf file type

Adding some custom css would reduce the height of the thumbnails. This code can be added to your theme’s functions.php file:

add_action('admin_head', 'mlf_custom_css');

function mlf_custom_css() {
  echo '<style>
    .media-attachment, .media-attachment img, a.tb-media-attachment img, a.media-attachment img {
      height: 135px !important;
    }
  </style>';
}