I emailed a final build, digitally-signed APK to a client. Should they be able to open it from their email, or can you only open signed APKs in a shell like mentioned here (or from the Play store of course)? They’ve been able to install APKs before now just from my emails, but they’ve never been digitally signed.
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If the client gets the email on their phone, they should be able to open the .apk directly from their phone to install it.
You do need to allow the installation of Unknown Sources in Settings => Application
This is how a user installs the Amazon Marketplace, too. Amazon sends you an email where you can open the .apk file.