Acer Aspire S5-391 User Manual page 77

A code displays on your computer, which should match the code
displayed on your device. Select Yes and click Next. Then, accept the
pairing from your device.
Note: Some devices using older versions of the Bluetooth technology
require both devices to enter a PIN. In the case of one of the devices
not having any inputs (as in a headset), the passcode is hardcoded
into the device (usually "0000" or "1234"). Consult your device's user
manual for more information.
Using a Bluetooth connection - 77