You can "NOINDEX" PDF files but the process is a bit different; since PDF files don't have a section for you to insert the
meta robots tag that you mentioned, you have to do it using the
x-robots noindex command, which is an HTTP header command that you add to your .htaccess file.
This article describes this process in a very clear way - if you know how to work with .htaccess files it should be simple to implement this (but be careful if you don't).
Edit: as mentioned below, depending on the server you use you won't have access to an .htaccess file but the logic is still the same: you need to do it through HTTP headers.