WordPress – PHP list of PDF files

I’ve written this code:

function userfunc_get_file_links($sub_folder) {
    if ( is_user_logged_in() ) {
        $directory = get_stylesheet_directory()."/files/".$sub_folder;
        $scanned_directory = array_diff(scandir($directory), array('..', '.'));
        echo "<ul class='support_links'>";
        foreach ($scanned_directory as $key => $value) {
            echo "<li><a href='files/".$value."'>".$value."</a></li>";
        }
        echo "</ul>";
    }
}

I’ve added it to my functions file, and I can successfully apply it with a page template and it displays a list of file links.

(Template file):

<?php
/*
Template Name: supportpage
*/
?>
<?php get_header(); the_post(); ?>
    <div id="content" class="inner-page">
        <div class="main-content">
            <div class="brack-cum">
                <div class="inner-small">
                <?php 
                   if (function_exists('wp_bac_breadcrumb')) {wp_bac_breadcrumb();}
                ?>
                </div>
            </div>
            <div class="title">
                <div class="inner-small">
                    <h2><?php the_title(); ?></h2>
                </div>
            </div>
            <div class="content-page inner-small">
                <?php
                    the_content(); 
                ?>
            </div>
        </div>
    <?php get_footer(); }?>

However, when I click them it brings up a 404 error and I can’t work out why. I’ve tried resetting my permalinks and checking the FTP path (its 100% correct).

If I try moving my /files/ folder to the root directory of the server with

$directory = home_url()."/htdocs/files".$sub_folder;

It returns

Warning: scandir(http://localhost/wordpress/files/application_notes):
failed to open dir: not implemented in
C:xamppappswordpresshtdocswp-contentthemestwentythirteen-childuser_functions.php
on line 5

The files are pdf files for the most part (“how_to_do_stuff.pdf” etc).

Any ideas on the steps I could take from here?

EDIT1: With the original code displaying the links but returning a WordPress 404 on click:

var_dump($directory)

string(93) "C:xamppappswordpresshtdocs/wp-content/themes/twentythirteen-child/files/application_notes"

var_dump($scanned_directory)

array(3) {
  [2]=>
  string(9) "test1.pdf"
  [3]=>
  string(9) "test2.pdf"
  [4]=>
  string(9) "test3.pdf"
}

With the code that produces the warning:

var_dump($directory)

string(57) "http://localhost/wordpress/htdocs/files/application_notes"

var_dump($scanned_directory)

NULL