To be able to have other users be notified of changes like this you either need to have the server send a notification to the client or you need to have the client polling the server for updates.
Having the server notify the client is the most efficient and quickest and you would probably want to use WebSockets. I haven't used WebSockets with a PHP server before but a quick search shows maybe you'd use a library like this
. Also here's a tutorial
for a WebSocket chat application which seems similar to the functionality required by your web app where a message is sent from one client and received right away on another client.
If you want to poll then you can just send an ajax request to your server every few seconds and your server would have to remember recently saved annotations and return a response with that information.
To display a message to the user you could take a look at the code that is used in ReaderControl to display a message when annotations are saved. If you look in ReaderControl.js in the saveAnnotations function there is an overlayMessage element that is shown as a jQuery UI dialog. In the success and error callbacks you can see the setting of the data-i18n attribute. This is just for the internationalization library WebViewer uses so you could instead just set overlayMessage.text('my message') or you could modify translation-en.json yourself to add more translations.
PDFTron Systems Inc.