What PDF program do I need for a Mac (OS 10.11.2) to be used with a Fujitsu Scansnap model ix500?
You can use the available software from the official website in order to scan to PDF with your device. There are available on the support page where all the versions for PC and MAC are hosted.
Support page: http://www.fujitsu.com/global/support/products/computing/peripheral/scanners/scansnap/software/ix500.html