Today I helped someone with the same issue.
They came to me asking about this app they can't uninstall, and to me seems like it's a malware that came out of an official Play Market store. They tried installing an email app, and that email app required you to download this PDF Ai: Addon app, outside play market.
Once it has been installed, you couldn't uninstall it, as every time you try to hold down the app to pop the menu to uninstall it flashes back to main screen to prevent itself from being removed. If you try to go in Settings > Apps > [app name] and try to uninstall from there, it will send you back to home screen again.
Tried going into settings to reset it's full control over the phone permissions, and that was blocked as well, it sends you to main menu on the phone. Then I tried installing an app to remove it (CCleaner), and when you try to open it it closes the app. Very nasty malware.
Then I noticed it also not letting you Power off the phone or restart, as it turns off your screen when the power off button show up.
I found out you can remove it in Safe Mode, but i needed to figure out a way to get in safe mode, as I noticed the buttons cobination didn't work.
So here is my solution. First of all, find out if your phone model has a specific way of getting into safe mode (for example, a button combination to restart in Safe mode).
What I also noticed when trying to troubleshoot this problem was the fact that after a forced phone reboot, when you open your phone (after the lock screen, when it says "Phone is starting...." you can see something flashing for a second at the beginning, meaning that malware initialized. I also noticed that the malware only initializes after you went away from lock screen (after a fresh restart). Meaning that after a phone restart, you can access your power off buttons.
So, for the solution, first of all, you need to forcefully shutdown the phone, the way i did it was to hold down POWER button and both Volume buttons, and just keep holding until the phone eventually shuts down. Once it's back online, Do not unlock the screen, instead hold the power off button until you get the power off/restart option, and then hold down on the Power off option on the screen, until it shows "Safe Mode". Click on that option and it should let you restart your phone in Safe Mode. From there try to uninstall the app again, it should definitely work.
Hope this helps someone.
