How to prevent google from indexing pdf

Add to your robots.txt file:

User-agent: Googlebot 
Disallow: /*.pdf$

Using WordPress, easily use Yoast Seo -> Tools. If you cant find ‘File Editor’ under Tools, then your wordpress installation has ‘disablefile editing’ on.

Then you have to upload (or edit) the robots.txt to the root folder in your wordpress installation.

Check your robots.txt using: www.yourwebsite.com/robots.txt