Push Notifications are available via the Server API, so your best bet would be to enable this via Cloud Script. So for example, if player 1 gets the leaderboard around his own score and uses that to randomly select another user with a slightly higher score to challenge, you could use the PlayFab ID of that user, from the leaderboard data, to send the Push Notification. Obviously, Push does require that the player opted in to receiving those notifications, and that the player didn't subsequently turn off Push for the app in the device (which we would not get a notification about), so I would recommend also having a more traditional matchmaking system.
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