How do I stop PDFs being indexed by search engines?

Hey @makoo1, have you implement it?

In robots.txt use Noindex as well with disallow
like;
Disallow: /page-one.html
Noindex: /page-two.html
OR
can implement CAPTCHA option as well.

use "Disallow" to stop from indexing. You can use also Meta Robot Tags to disallow or allow page or file for indexing. Just use disallow in tags to stop indexing page in which your pdf file is.