Can my father's smartphone be compromised? My father said to me that he allowed installation of apps from unknown sources when he was asked to do it to go further with something (he said that he don't remember when or what he was doing). 

In android 8.0 (the version of his smartphone) we can allow third-party installation in individual apps, so I went in this tab and I saw that only Google Chrome was able to do this type of installation. So, I navigated through my father's apps and found nothing suspicious. 

When I went to files, I found a folder called didi. Inside, a file with no extention called psnger_encrypted or something like that. When I went to select the file to see details, I accidentally clicked on the file. Android said to me that it didn't find any app to open that file.

Could a file have been executed even when Android says that message? If this file is malicious, is it possible that it was executed and so infected the phone?

Doing some research, I found that a Chinese app called "DiDi" that have a file called "psnger" too. Also, I found a file in my father's phone called .omega.key, and this file is also seen in topics related to DiDi app. The fact is that I can't find this app on the phone, and I'm looking for ways that can indicate if the phone have any malicious content. I executed AVG, and Malwarebytes but both detected nothing. What else can I do?
The correct approach in case of a virus/malware infected device (even if it already had been clean-up by e.g. Google) is to totally erase the device (user and system data) by performing a factory reset and re-flashing the complete firmware.

Virus scanner apps are next useless on Android as they usually don't have system access. And even with system access it is still easy to hide from Antivirus software.

The main problem is that the security patches are more than one year old. Therefore a malware app could have exploited the device and installed itself into the system. Therefore there may be still active components in the system that are invisible to you and any antivirus software.

I would also not to hurry up to get the device back in a usable state. People (especially relatives) learn better not to click on everything they see when there are negative consequences.