Prevent Google and other search engines to index PDF / media files
Simple answer: this is not possible, because when you load the URL of a media (a PDF, a JPG…), WordPress is simply not loaded, neither SEOPress.
Your web-browser is directly opening your media, nothing more.
How to secure sensitive media files?
The minimum protection is to set the page where you have the link to your PDF in nofollow. But, if someone embed your PDF link elsewhere, Google will index it.
The only way to truly protect your PDF is using an authentification with PHP.
Do not host any private and sensitive media on your WordPress media library without any security!