Go to Settings > Security and toggle on "Install from Unknown Sources."
Instead of installing an APK like AppBoostGlobal, you can achieve better performance by: appboostglobal apk
Peppol Directory - an OpenPeppol AISBL service
Download data [BusinessCards XML | BusinessCards w/o doctypes XML | BusinessCards JSON | BusinessCards CSV | Participant IDs XML | Participant IDs JSON | Participant IDs CSV]