If your Play store App's cache memory is full or lack of space on your device for the download, you will get these errors. Here is the steps on how to fix the Google Play "Package File is Invalid" on Nexus 7.
* Tap Settings in the app drawer, scroll down to "Apps" and select "All".
* Tap Google Play and force stop it. Tap "Clear Cache", "Clear Data" and "Uninstall Updates".
* Exit every app and reboot the device. Then run a Google sync and try it again.