If you have a WordPress website, then what I discuss below is arguably the best way to secure your files so that they're only accessible through your website, and prevent them from being passed around and used by unauthorized users, including search engines:
1) Store your files on Amazon S3.
2) Use a WordPress plugin for Amazon S3 like S3MediaVault.com to then securely embed expiring, secure links to those media files on your WordPress site.
3) If you wish to protect the page itself (where you have the download links, or video or audio player), then use a WordPress membership plugin like DigitalAccessPass.com.
4) This is not a true Digital Rights Management (DRM) solution - so once someone downloads your Video, Audio or PDF to their computer, you no longer have control over it.
But that's OK because 99% of the abuse happens because...
a) it's not locked down,
b) because people can easily pass around links,
c) it gets mistakenly indexed in search engines,
d) someone shared it illegally in a forum, Facebook group, etc.
5) Your actual paying users/members will rarely, if ever, illegally share stuff that they've paid for. Even if they do, it may be with a friend or a family member.
6) Those who pay for any content online, usually have a good mindset and ethics. They're not going out there looking to screw you (the creator) and pirate your content like crazy and make you lose your income.
7) Don't lock it down so hard that you make things difficult for 99% of your ethical paying customers, just so you can make things difficult for the unethical, unscrupulous 1% who will try to steal from you regardless - taking screencasts, screenshots, looking in black-hat forums and warez websites for passwords and crackz etc.
8) Focus on the paying members, and giving them maximum value for their money, rather than focus on the minority of losers who will never pay for anything, and will try their best to pirate stuff.
9) Also look for advanced features like HLS True Video Streaming (which S3MediaVault.com supports) to stream your videos and make them impossible to download.
Let me know in the comments below if you have any further questions.