The Apple Pencil is currently only compatible with the iPad Pro (9.7-inch and 12.9 inch models) because it requires a specific kind of display technology to work. The iPad Pro’s screen is equipped with such a subsystem that registers the Apple Pencil’s signal 240 times per second (which is twice as fast as reading a user’s finger) and allows it to read its position, tilt, and force.
“This data, combined with Apple designed software, means that there’s only milliseconds between the image you have in your mind and the one you see on the display,” Apple said.
The iPhone 7 and 7 Plus are currently not equipped with the display technology to support Apple Pencil.