Someone who's downtrodden is mistreated by some powerful person or group. An exploited, underpaid worker is downtrodden.
The adjective downtrodden is more often used to talk about groups of oppressed people, rather than one specific person. A history textbook might discuss the downtrodden masses rising up in a revolution against their rulers, for example. In the 1560s, downtrodden was first used to literally mean "stepped on," with the figurative meaning following soon after. Trodden, "that which is stepped on," comes from the verb tread.