Try settings - storage - explore - downloads
And select your downloaded apk. If that doesn't work, download file explorer and select it from inside there. Idk why it sees it as an apk this way and not when you select open or from the notification.
I downloaded a ton of APKs last night. When i found the apks in the file folder and ready to install. O was prompted to choose package installer or a different program
You should be able to just install the APK directly from your phone as long as you have unknown sources enabled. You can also install it using adb by using the following command: adb install filename.apk
I had the same problem at first, but I downloaded androzip and installed my .apks just fine. I only have two apps I side load old beta of fb and the chat enabler antaresone wrote to keep messenger in the native app.