How to change the target of a file for a multiple file field in drupal 8?
You can do this in a hook, but also in a custom file-link template in your theme. Check if the file link has a class specific to a pdf file and then set the target attribute:

file-link.html.twig

{% if attributes.hasclass('file--application-pdf') %}
  &lt;span{{ attributes.setAttribute('target', '_blank') }}&gt;{{ link }}&lt;/span&gt;
{% else %}
  &lt;span{{ attributes }}&gt;{{ link }}&lt;/span&gt;
{% endif %}