To do this you will need access to the Robots.txt file which will be at the top level of your website hosting folder. Your website administrator should definetly know where to find this.
You will just need to update the file to tell google not to index any pdfs for example. You can also be specific if you just want a few files not to be indexed.
An extra step would be adding the rel=no follow attribute to your links that go to the pdf
<a href="your.pdf" rel="nofollow">Download PDF</a>. Although google isnt indexing the download page it will still be crawling and looking for other links and pages that don't have exclusion rules.
Check out these pages for more info.