Android appends a sequence number to the package name to produce the final APK file name (it's possible that this varies with the version of Android OS). The following sequence of commands works on a non-rooted device:
Get the full path name of the APK file for the desired package.
adb shell pm path com.example.someapp
This gives the output as: package:/data/app/com.example.someapp-2.apk
.
Pull the APK file from the Android device to the development box.
adb pull /data/app/com.example.someapp-2.apk
The location of APK after successful pulling will be at ../sdk/platform-tools/base.apk on your pc/laptop.