Hi there,
This is a bit out of scope of PDF.js Express, but we can offer a suggestion.
The best way I can think of to accomplish this is to use WebSockets. The most popular library for using WebSockets is ws. WebSockets are real-time connections between server and client. So what you can do is listen for changes on the client (page changed, zoom changed) etc, and send these events to your WebSocket server. Your WebSocket server would then broadcast these events to any other connected user.
Another solution would be to use Firebases realtime capabilities. You could create a new firebase document for each PDF, and in the document save the state of the user (zoom, page). Any other user could subscribe to changes to that firebase document.
I hope this helps a bit!
Thanks,
Logan