Hi Madeline,
Sorry about that. This could be either of two problems. If your EndNote library is stored on a network share, a cloud service folder such as DropBox or iCloud, that could be causing the problem and even corrupting your library. An EndNote library should be stored locally in your documents folder.
The other issue is how the Sierra OS handles PDF’s. Apple has re-worked their PDFKit software, causing any application that utilizes PDF’s to become sluggish and even crash. We’re waiting for a fix from Apple on this issue. A work around for EndNote would be to move the PDF folder out of your library’s .data folder.
I hope this is helpful.