I'm going to use a plugin called PDF Stamper for an ebook that I"m selling. The plugin requires me to upload the pdf to the normal me uploads folder: i.e. http://www.domain.com/wp-content/uploads/2011/10/document-file.pdf I was under the impression that this folder is picked up by search engines. Which means my ebook isn't really behind a "pay wall." So if someone shared the pdf url path, they could access the pdf without paying. I tried searching online, but I was unsuccessful in finding an answer!
You can do it easily by editing your robots.txt file and adding the following details: User-agent: * # disallow all files in these directories Disallow: /wp-content/
Use robots.txt to disallow search engine indexing. Add index.htm file to your folder to disallow user to see it.
All of above options are helpful, but it will not stop downloading the PDF, if someone knows direct url. Better option will be using some sort of shopping cart type plugin, or something that can set a session and some restrictions on the file. Or you can lock your PDF and give the password to your customers!