Oxygen OS apk installation Error : Cannot display pdf When I try and install apk file by clicking from Files App, I get the error message "Cannot display pdf (.apk is of invalid format.)"

The system tries and opens pdf reader app, instead of package installer app.

When trying to install from Downloads App, I get the message "can't open file".

The phone is a new OnePlus X phone with OxygenOS 2.2.1, Android Version 5.1.1

In the Settings, I have checked the Unknown Sources App Installation Option.

Also I cannot see the Developers Option in Settings.
The developer options become available after you click the build number 7 times in the about section of your phone.