The attempts to display the PDF in a hidden iframe. On some systems that will act as a hack to cause it to be downloaded.
It won't work on iOS because Mobile Safari doesn't have any concept of "Downloading PDFs". The only way to download a PDF in Mobile Safari is to:
1 Open it in the browser normally
2 The user clicks the "Share this" button
3 The user picks an application (such as Good Reader) which will store the PDF in its storage area
If you want a reliable "Download a PDF" feature with a sensible fallover position for browsers which don't understand downloading, then use
the Content-Disposition header and a regular link.