The pdf preview in the main window was useful because it made accessing your pdfs very quick and easy instead of having to open them all in separate windows which can result in a lot of tabs. If you have a large monitor and increase the size of the preview window, I find I can read the pdf well and highlight too. You always have the option to open a pdf in a second window if you need that extra size.
I’ve reverted back to X9 because I really loved that feature but hopefully they bring it back because I do like the visuals of Endnote 20.