From your question,  it's likely that you are using Android Plus device agent and you want to silently push (via .pcg some app (likely from Google Play store) which you don't have the apk.   This has been answered many times in this forum.  It is technically possible to download apk from some unofficial app stores/archives.  However, I personally won't recommend this approach for production devices because the apk file may have been tampered with malware and the process is likely illegal in the strict sense.  Many years ago, some of my customers used back-up tools to create apk file from app directory of device installed from Google Play store. I also won't recommend this approach as the back up apk often do not include all the files as the original apk and there can be compatibility problem if the apk file created is used for installation on devices of different model/firmware version etc.  The only legal way to get Google Play store app properly and legally installed on Android+ device is to use app-catalog rule(s), but "silent" push is not possible.

If your devices are enrolled with Android Enterprise device agent and associated with a managed Google Play account, then you just select apps from the Managed Google Play store hosted on Google server, and then add the selected app as mandatory application in an Android-Enterprise app-catalog rule for the app to be "silently" pushed.  This is one of the major advantage of using AE rather than Android+.