As mentioned above, lat pulldowns primarily target your back muscles, but they also work your shoulder joints and upper arm muscles.
This is why a lot of lat pulldown machines can also be used for other exercises, like seated rows. This can help you work a broader range of related muscles to fill out and tone the upper body more effectively.
In terms of targeting the shoulders specifically to increase mass, one of the best ways to do it is with the overhead press—typically a free weights exercise you can do with a set of dumbbells.