7. PDA is a portable computer (can / be used / as a mobile phone, the Internet browser and a little media player).
В данном случае, необходимо использовать слово that или which, так как речь идёт о компьютере.
Предложение с вставленным словом: PDA is a portable computer that can be used as a mobile phone, the Internet browser and a little media player.
Ответ: PDA is a portable computer that can be used as a mobile phone, the Internet browser and a little media player.