If you want to add a download link. When click this link then any type of file,image, pdf will be download.You can do this using a simple php script. Yon can follow this code

First Create a simple html file where you want to add download link.

<a href="download.php?file=techpark_photo.jpg">Download</a>

i want to download a picture name “techpark_photo.jpg” so parameter is file name “techpark_photo.jpg”.

Then create a php file name “download.php” which you use in above file.

$file = $_GET["file"];
if (file_exists($file)) {
header('Content-Description: File Transfer');
header('Content-Type: application/octet-stream');
header("Content-Type: application/force-download");
header('Content-Disposition: attachment; filename=' . urlencode(basename($file)));
// header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize($file));

Important thing is that picture and script same folder if you change this then you can use your folder directory.

If you face any problem then please inform me, i will try to help you.