How to give more security to APK file

if proguard is used right, it obfuscates your apk, so all your classes and methods are named like a.aa.aba.a(x,xx,xzzx) and no one can read it.
Also release-sign your apk with a keystore and private key. You need to do that anyway before you can put your app to the play store.

Please follow the guidelines by Google.
https://developer.android.com/studio/publish/app-signing.html