EndNote X7 is incompatible with macOS Sierra
A new version of the mac OS, version 10.12, called 'Sierra' is due to be released on September 21 2016 (see: http://www.apple.com/au/macos/sierra).
If you open EndNote for Mac you should see a pop-up window that explains that there are issues with EndNote's PDF viewer and Sierra.
These issues have been reported to Apple, but in the meantime it is recommended that you DO NOT upgrade to macOS Sierra.
For more detailed information see the Thomson Reuters EndNote Community.