As we PaidFreeDroid provide APKs of apps, some user are unable to install as it is non-market. Well this is a simple problem related to android settings.
So now inside the security option on your Android device you will find a option that states “Unknown Sources“ well you need to make sure that option is checked as it simply allows your device to install application that are not officially from the Play Store, so if you are installing applications from your SD Card or directly from somewhere else then this option will not allow that installation.
So this was the real problem and it was solved after this option was checked. So now if you are the one who is facing the same problem do make sure that this option is checked in order to install the third party, unofficial, unknown sources app apart from the play store easily onto your Android Device.