I was wondering if there's a way of writing an htaccess rule that would block direct access to any directory in the site that doesn't have an "index.php" file

so :

http://www.mysite.com/pdf/mypdf.pdf would work
http://www.mysite.com/pdf/index.php would work if there was an index.php in the directory
http://www.mysite.com/pdf/ would redirect to the site root

or am I not approaching this problem in the right way ?

thanks for any help you can give