An APK file is an app created for Android, Google's mobile operating system. Some apps come pre-installed on Android devices, while other Developer : Google. APK files are meant to be used by the Android OS to install an application. APK files are basically just a wrapper around an app, you can also open one in a program like WinRAR or some other unzipping/decompression. Anyone have a solution for this everytime I try open a downloaded file i get an error message which simply says: cannot open file If i send apk as an attachment via Sent from my SCH-I using AC Forums mobile app.