If you have a Members Only section with PDFs embedded or linked, despite the section being password protected, the files themselves aren't actually secured. Any media file uploaded to the media library can be viewed in a google search. The files themselves are public even if the pages are inaccessible. But that's only if someone has the link or knows to look for the link. There is one option you can try if you have adobe acrobat. You can password protect the PDFs. So the PDF would still be searchable, but only those with the extra password can actually view it.
If you do not wish to add another password to these files, there is an option using Yoast SEO.
1) Go to the Media Library and click on the pencil button (top left corner) of the PDF you want to edit. You should now see the Attachment Details.
2) On the right side of the popup, scroll down till you see Edit more details.
3) You should now be on a page that says Edit Media (looks like a Post).
4) Scroll down till you see the Yoast SEO panel and click on the Advanced Settings.
5) Disabled "Allow search engines to show this Media in search results?" and "Should search engines follow links on this Media?"
Please keep in mind though. While this method would hide these files from search engines, it still doesn't secure the files from being seen by someone without signing in.