How do you display a file asset (PDF) in the template?
What do you mean by 'display the asset'? Showing a thumbnail is not an option, because no thumbs are made for PDF files. If you'd want to show the filename, for example, use the filename property of the asset.

If the page contains multiple entries, you'd have to loop through the entries first. Then, for each entry, check if it has an asset in the desired field (by using | length, after which you can display the asset.

{% for entry in craft.entries.section('bla').relatedTo('bla') %}
    {% if entry.dataCard | length %}
        {% set asset = entry.dataCard.first() %}
        &lt;a href="{{ asset.url }}"&gt;{{ asset.filename }}&lt;/a&gt;
    {% endif %}
{% endfor %}