Disable opening app in app When I am in the file browser and open a PDF, the PDF viewer will be shown as the file manager in the list of the apps. When I press back in the PDF viewer, it will go back to the file manager eventually.

Is there some way to make the app open on its own, with the given file as a starting point?
There is an Xposed module that does this, it's called ActivityForceNewTask.

Assuming you're rooted, install the Xposed Installer, install the framework from within the app, reboot, download the module, enable it, reboot again.