Install application without Google Play I made an application, and I want my friend to install it from a link, but the apk file cannnot be installed on the Android phone. He has to use some file explorer to install it. But I see many apps can be installed from a link and installed directly without using the file explorer. 

What should I do to my APK? Is it a signature problem?
Normally phones should be able to install apps even if they are not signed. Normally your friend should be able to install your app. I was able to download the application, and I could also start the installation from the link.

Make sure that your friends phone has 'unknown sources' enabled in the security menu (enable Settings > Security > Unknown Sources). Because otherwise it will also not work (without using Google Play == unknown source).

No offense though, but I didn't test all the way through the installation, because I don't trust it well enough and I have no idea what the app actually does. For all I know it is a virus ;-)