If you open PDFs in different windows and accidentally close the library, there is no way to reopen it. The only solution I've found is to close each PDF and click the Zotero icon again, which reopens the library, but there's no way to reopen it at any time. You should add a button or feature to do this.

Theoretically, when you have a PDF open, there is a "view in library" button, but if the window with the library is closed, nothing happens anyway.

I'm using Zotero 7.0.1 on MacOS.