WordPress is a free and open-source content management system written in PHP and paired with a MySQL or MariaDB database. Features include a plugin architecture...
Configure the file response to include the Content-Disposition HTTP header with the value attachment, while keeping the correct content type, application/pdf. You can do this in the application that serves the document or through a web server rule limited to the relevant files.
For a link to a file on the same domain, the HTML download attribute is a simple alternative, although behaviour can vary between browsers. Test the download on both desktop and mobile. If the document is private, check access permissions separately: triggering a download does not prevent someone from sharing the link.