The only plugin that gets close to what you want is this one. To protect a PDF file with a password you need to use HTTP password protection (WordPress can't do it itself). This plugin adds a password to your entire blog, however it could be fiddled with to only protect a directory (for instance /path/to/pdf/folder).
If the link to the file is in your members area though, you probably don't need to password protect it. The only people who can find out the link are the people who have a member username/password, and if they're willing to give out the PDF link to someone, they're probably just as willing to download the PDF and attach it to an email.
A final alternative solution would be to password protect the PDF itself, and write the password next to the link in the members area. You can password protect a PDF by clicking Security Options or similar in most versions of Adobe Acrobat Professional or when printing as a PDF on a Mac.