by default, you are using link-template-default.php for link template and page-template-default.php for the page template. you can follow this doc, https://www.wpdownloadmanager.com/doc/template-files/ for back-up your files, As documentation, you have to copy the file in the theme directory naming a folder called download-manager, then paste it in tpls>page-templates for page template or tpls>link-templates for link template.
For opening in new windows, please add this code replacing [download_link] in page/link templates PHP file as i described above,
<a href="[download_url]" onclick="window.open('[download_url]','newwindow','width=1200,height=800'); return false;" class="btn btn-primary" target="_blank">[link_label]</a>
thanks
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