I don't know why we haven't done this by default. I'll make this change in the next update.

Meanwhile you could do it by opening file:

stachethemes_event_calendar\view\front\tabs\intro.php lines 109 and 114

add target="BLANK" to both <a> tags.

<ul class="stec-layout-event-inner-intro-attachments-list">
    <li class="stec-layout-event-inner-intro-attachment stec-layout-event-inner-intro-attachment-template">
        <div>
            <p class="stec-layout-event-inner-intro-attachment-title stec-layout-event-title2-fontandcolor"><a target="BLANK" href="#stec_replace_url">stec_replace_filename</a></p>
            <p class="stec-layout-event-inner-intro-attachment-desc stec-layout-event-text-fontandcolor">stec_replace_desc</p>
        </div>
        <div>
            <a target="BLANK" href="#stec_replace_url" class="stec-layout-event-title2-fontandcolor"><?php esc_html_e('Download', 'stec'); ?></a>
            <p class="stec-layout-event-inner-intro-attachment-size stec-layout-event-text-fontandcolor">stec_replace_size</p>
        </div>
    </li>
</ul>