Example 2 on w3schools shows what you are trying to achieve.
<?php header("Content-type:application/pdf"); // It will be called downloaded.pdf header("Content-Disposition:attachment;filename="downloaded.pdf""); // The PDF source is in original.pdf readfile("original.pdf"); ?>
Also remember that,
It is important to notice that header() must be called before any
actual output is sent (In PHP 4 and later, you can use output
buffering to solve this problem)