Why can't I start a an email or document on my iPhone and pick up where I left off on my iPad?
Handoff is part of the Continuity feature Apple introduced with the launch of iOS 8. The feature allows iOS users to edit documents across multiple devices, receive calls or texts on their iPad and iPod touch, and more. All your devices must support and currently be running iOS 8.0 or later to use the Handoff feature. This allows you to start an email or document on your iPhone and pick up where you left off on your iPad. Your iPad also has to be connected to the same Wi-Fi network as your iPhone, and you must be logged in with the same Apple ID on both devices. Also make sure Bluetooth is enabled on all of your devices.
If Handoff still won't work on your iPad, navigate to Bluetooth on all of your devices and make sure the feature is not disabled.