PDF Expert (for Mac) and PDF Expert (for iOS devices: iPhone, iPad) are available as separate purchases due to the fact that they run on different platforms. You can purchase one copy of PDF Expert 6 and use it on multiple iPhones/iPads provided that you have the same Apple ID on all devices. As for PDF Expert for Mac, you can apply one license to three Macs in case the app was purchased from the PDF Expert website and use it on multiple Macs, provided you have the same Apple ID on all of them.