You'll need to modify 2 files.
1) Modify the file /controllers/front/AttachmentController.php
Find the following line header('Content-Disposition: attachment; filename="'.utf8_decode($a->file_name).'"'); and change it to header('Content-Disposition: inline');
2) Open the file /themes/YOUR_THEME/product.tpl
Find the following line <a class="btn btn-default btn-block" href="{$link->getPageLink('attachment', true, NULL, "id_attachment={$attachment.id_attachment}")|escape:'html':'UTF-8'}"> ~line 550 and change it to <a class="btn btn-default btn-block" target="_blank" href="{$link->getPageLink('attachment', true, NULL, "id_attachment={$attachment.id_attachment}")|escape:'html':'UTF-8'}">