If the file has been uploaded to a public location on your web server then you’re essentially bypassing download monitor (and wordpress entirely) by linking to the PDF directly.
You need to upload the PDF to a private location (non-public or protected by .htaccess for example) and enter the local path to the file within download monitor. You can then provide users with the download monitor specific URL in order to download it (e.g. http://yourwebsite.com/download/10/)
