I have a link to PDF document on a public web-page. How do I prevent search engines from indexing this link and PDF document?  The only idea I thought of is to use CAPTCHA.  However, I wonder if there are any magic words that tell a search engine to not index the link and PDF document?  Options using PHP or JavaScript are also fine.Just to make it clear.  I do not want to encrypt PDF and protect it with password.  I just want to make it invisible for search engines, but not for users.
How to prevent a PDF file from being indexed by search engines?
indexing;pdf