Aghh! I just updated to Endnote 20 and have exactly the same gripe! The whole point of the preview function for me was so I could pull up my library and quickly check the reference. As much as I like the new PDF annotator, it was always more useful to have it in the window.
While I’m adding my voice to this discussion it would be amazing if we could get a mac+F shortcut to search within PDF files, either in the break-out pdf viewer or in the preview. Save a few clicks which equates to hours over the year.
I’m downgrading to X9 for now, but would would really like to see these new features in 20 soon.