Can’t download krypton…help!

Please help. I've downloaded Krypton using ARM. When I try to install it from the download, Package Installer is the only "open with" option that populates.

When I select Package Installer and try to install Krypton, I get "app not installed". I'm running android 5.1

I've downloaded APK installers from the Google App store, but no matter what I only end up with the Package Installer option. I have no idea what to do.