This problem of Endnote freezing persists with X8.0.1 and macOS 10.12.6.
There are strong indications that the PDF previewing is the culprit, regardless if the PDF file has lots of annotations or not.
I have for now switched my my view in local library mode to not show any PDF page, meaning these are the settings “bottom preview”, “right preview”, and “references only”.
So far this has increased stability, but I cannot say for sure that this is a fully functional workaround against freezing. There will be moments also when we need to verify any new attachments to be correct :-/
I open PDFs via the keyboard shortcut opt-cmd-P that launches the configured external PDF viewing app.
I am not surprised that Apple (on their high horse) is not giving high priority to fixing the PDFkit framework. See also:
https://tidbits.com/article/17152