I am creating a web site to sell digital articles which are in PDF format and are encrypted.

The solution I am thinking is to let buyer download encrypted PDF file from my web site and then I email him/her a password for him/her to open and read.

I have two concerns here:
1) How can I identify each buyer's computer after they download the article? The purpose is not to allow the same buyer to download from computer B after download from computer A and try to open the file from computer B with the same password as computer A. The goal is to limit buyers to share password with friends. Even his/her friends can download the encryped PDF file, they can not open it with the password.

2) How can I limit a buyer to copy/paste out the content of my PDF file? That is how to disable the 'select' and 'copy' menu of the PDF file? My original file is a MS Word file. When I convert Word to PDF, is there any setting to change to disable 'select' and 'copy'? 

Or is there better solutions?

Thank you for your wisdom on this.

-Leo