IOS and Android push notification based on file access

I need to find a service that will provide push notifications (or something like that) to users of my mobile app when they share a file (such as a video, PDF, etc).

Use Case:

• User Ann loads my app
• The app shows a list of videos and PDFs
• Ann can share any of the files via the OS's built-in share system
• Ann shares a video (for example) with their fried Bob
• When Bob starts to watch the video, Ann gets a push notification (or something like it) that Bob has just started watching the video she shared
• Ann also gets a notification when Bob is done watching the video
• Ann also shares and PDF with Frank and gets notified when Frank opens the PDF

Currently my app shares files using the URL of the file located on my server. I'm looking for some sort of service where I could host files that would provide push notifications (or something like them) to the user who shares the file.

Is there a service or a library out there like this? It has to work on both IOS and Android.

If there isn't a push notification system answer, is there a way to alert via email?

Thanks!