THIS code looks right, BUT >1 should probably not be there
THE main image is 0, the next is 1. So you would have to have 3 images for a display to happen
try this
<?php
//Pro Hack PDF to position
if (!empty($product->images) {
?>
<div class="additional-images">
<?php
// List all Images
if (count($product->images) > 0) {
foreach ($product->images as $image) {
// Displays ONLY pdf files
if ($image->file_extension ==pdf) {
echo '<div class="floatleft"><strong> Télécharger la brochure : </strong> <br>' . $image->displayMediaThumb('class="product-image"', true, 'class="modal"', true, true) . '</div>'; //'class="modal"'
}}
// PRO DONE, added the close tag
}}
?>