Alignement

Yes thank you: I’m starting exactly from that document. I added to the CSS the following:

.sdm_download_link{
    text-align: center;
}

but the result is null: http://www.nuove-vie.it/unambigua-utopia/
Any new idea?

create a new post and add the above code but don’t publish the post.

Mmm! Cannot understand exactly what you are asking. I made this already, of course. I tried it on posts and pages.
When seen via the WP editor the shortcode appears to be centered, but this because a shortcode is Text! While the <div> doesn’t work at all.
Do you think that the culprit is the theme? And how can this be?
http://www.nuove-vie.it/soldatessa/
This is a new link, published today.

I had some trouble to obtain what you’re asking for, but in the end I obtained this page in Twenty-Sixteen style:
http://www.nuove-vie.it/backup/3755-2/
Around the double shortcode, the fancy=”2″ block, and the download counter I’ve set the centered <div>.

<div style="text-align: center;">[sdm_download id="xxxx" fancy="2" new_window="1"]</div>
<div style="text-align: center;">[sdm_download_counter id="xxxx"]</div>

You can see that the download image and button is not center aligned, while the download counter is.
So (apparently) the theme is not the culprit.