How to stop Google from indexing Media files?

you could put a robots.txt file on your server:

https://www.robotstxt.org/robotstxt.html

even google has a robots:txt:

https://www.google.com/robots.txt

@danthefan I was referring to file uploads in the Media Library. So do I add an entry in robots.txt file like below?
Disallow: /wp-content/uploads/2018/09/holiday.pdf

That means I have to add individual ‘Disallow’ lines for each media upload?

You can allow or disallow by directory or file ending too.