FYI, using the Mod_Rewrite Variables Cheatsheet makes this example, and all advanced .htaccess code easier to understand. This demo lets you set a cookie with 1 of 3 values, then you just request the pdf file with a normal link click and get 1 of 3 different responses. This is accomplished with a nice bit of .htaccess code.

Here's an example of validation links from the AskApache footer.

RSS | XHTML 1.1 | CSS 2.1

document.getElementById("validat").innerHTML += ' | <a href="http://feedvalidator.org/check.cgi?url=https://www.askapache.com/feed/">RSS</a>  | <a href="http://validator.w3.org/check/referer?ss=1;outline=1;sp=1;debug">XHTML 1.1</a> | <a href="http://jigsaw.w3.org/css-validator/check/referer?warning=0">CSS 2.1</a>';