Workers at this factory complain that they are underpaid.

underpaid 是动词 underpay 的过去分词形式。underpay 由前缀 under-（表示"不足、低于"）和动词 pay（支付）组成，意思是"支付不足、少付工资"。underpaid 表示"被少付工资的、工资过低的"。