How do you use WP slugs for PDF files / media?

In my opinion it is best to add an endpoint of the form mysite.com/pdf/<id> or something similar to your like. That way you avoiding attaching the attachments to posts/pages or create a page for each pdf file with is redundant imo for your needs.

This is a very small and basic guide. You can modify to your needs.

First of all we add an endpoint matching all the request of the form mysite.com/pdf/<id>. Note that you should flush the rewrite rules for the new endpoint to be working. You can do it manually by going Settings->Permalinks->Save Changes or by using flush_rewrite_rules. It should be executed either on the shutdown hook, or on plugin/theme (de)activation. Executing it on the init hook is bad practice.

function my_endpoints() {

    add_rewrite_endpoint( 'pdf', EP_ROOT);

}

add_action( 'init', 'my_endpoints' );

Then we use template_redirect to handle the request.

function my_template_redirect() {

    global $wp_query;

    /* If it is a /pdf/<id> request then serve the file */        

    if(isset( $wp_query->query_vars['pdf'])){

        // You can use sanitize_file_name also
        $pdf_id = intval($wp_query->query_vars['pdf']); 

        /* Find the file and serve it */

    }

    return;
}

add_action( 'template_redirect', 'my_template_redirect' );