When people send me an email, especially if it has an attachment, they don't know if I've got it, or if I'm taking action on it, unless I tell them.
It's a basic professional courtesy to keep others informed; and you'll get into less trouble with frustrated colleagues if you set expectations about what you'll do, and when you'll do it. Then deliver on those expectations.
My rule of thumb is to keep the conversation moving at a reasonable pace. In principle I reply to everything I receive so that the ball is back in the other's court.