You could redirect if it is the incorrect referrer page. In other words, they would have to come from a certain page, or it would redirect to page where they would to fill out a form to get content. I created this example for you. Just add it before closing </body> tag on the page.
<script>
window.onload = function() {
if (document.referrer !== "CHANGE THIS TO THE PAGE THAT LINKS TO PDF") {
document.location.href = "
CHANGE THIS TO THE PAGE THAT LINKS TO PDF";
};
};
</script>
So it should look like this:
<script>
window.onload = function() {
if (document.referrer !== "http://example.com/landingpage") {
document.location.href = "http://example.com/landingpage";
};
};
</script>