Using .htaccess and HTTP_REFFERENCE

Anyone here who knows how to use .htaccess well?

Im trying to make it so that people cannot view one of my pdfs directly by typing in its address, but has to go through another one of my pages.

Basically, I would use javascript, but you cant really put javascript onto a pdf. Anyone know how to do it using .htaccess? I tried to use HTTP_REFFERENCE, but it didnt work and i dont think thats how its used.

I did something like this for a site once.

I stored all the pdf's outside the web root directory so no one could access them, then serverd them up through a login interface.

Involves mime-types and the like.

how do u do that?