For the latest version (2.0.0): There is supposed to be a setting for (1), but the note in the supplied settings file seems to be misleading. I find that if I set `should_launch_new_instance` to 1, then I can have multiple PDFs open in Sioyek simultaneously, but only if I open the PDFs after the first from the termiinal; opening them from within Sioyek or from dmenu replaces the PDF that’s already open. But I never got any crashes. I use dwm on Debian.

I consider (2) desirable.