PHP to identify user.id

Hello,
I use Ultimate Member as a plugin to a casting/actors website to store all the talents data. I would like to add a Download button to download the talent photo/information using the E2Pdf plugin. At their support they told me to add some PHP code to the talent page, this one

$user_id = ‘1’;
echo do_shortcode(‘[e2pdf-download id=”1″ dataset=”2524″ user_id=”‘.$user_id.'”]’);

and use it to display the button. It works, as you can see here (use the download link):
https://wp.trentinocasting.com/?page_id=1854&um_user=ossvale
but as you can see, the $user_id variable is static (at this moment =1) but I would like to get the user_id of the current user being displayed in that page.
How can I get this info? And how in PHP?

Thanks

Valerio

The page I need help with: [log in to see the link]