Data compression began as a field of computing science dedicated to making data smaller while still preserving it, which, as it is apparent to the reader, is patently impossible (except through homeopathy, as discussed below). Naturally, if you want to make data smaller, you have to take some out, so that is exactly what file compressors do. Only the most dishonest retailers that offer compressors for insecure download actually claim that all your data will come out the other end.