Copyright Information from CLA
As you are being paid a fee, would you be the copyright holder, or would the person employing you hold the copyright? I don't know, may depend on your agreement with the publisher.
Someone could copy it, could collate it, could publish it. However if they did so without any changes, then I feel you could ask them to cease doing so, or come to an agreement that the source is acknowledged, or take action against them... though if that is financially viable is probably debatable.
Producing free downloads I feel is a good way to promote yourself... if each download contains some information relating to you as the author (maybe just your name and the date - though more info if the publisher permits it) then if they become popular, organisations that may wish to have similar things produced for them could ask you to produce that material. It is also something to add to your portfolio... you can tell people you produced the resources, how many monthly download there are, that kind of thing.
My view on it is that if you don't ever do things because someone might, at some point in the future do something with what you create that you don't approve of, you will never get anything in print (be it digital or traditional print).
Try posting in Legal if you need a legal view.