Would a person who doesn't know anything about technology be able to read an EPUB file without having to install any app? The phone might be in Android or iOS.
I suspect the answer is no, because I even have to install an app to read on my laptop. But I heard that iBook on iOS supports EPUB.
The file is converted from LaTeX, and will be sent via Facebook Messenger, if that's matter.
To read an ePub book, you need some kind of software that can open it and display it to you in the proper way, exactly like any other kind of file types.
I don't know about all phones, especially for Android ones, the set of preinstalled apps depends largely on the manufacturer choices.
Often, Google Play Books is preinstalled, but there are a lot of other ePub reader apps.
Anyway, I don't really understand what the real problem is. If you are going to send an ePub to another person that had no problem in finding and installing the Facebook Messenger app, you should not be concerned that he would not be equally able to find and install an app used for another purpose.
