I find the message below unhelpful. I never use the PDF Viewer. Is there any other problem using EndNote 7 and Sierra? EndNote for Mac: macOS Sierra Compatibility In preparation for Apple’s release of macOS Sierra on September 20, we have been testing various versions of EndNote. Through our testing, we discovered some issues with the EndNote PDF viewer. These issues have been reported to Apple, but in the meantime, we recommend that you DO NOT upgrade to macOS Sierra. We are not currently planning a fix to EndNote X7, but will continue to work with Apple to resolve these issues for X8. EndNote X8 should be fully macOS Sierra compatible.