if somebody discovering the how your dll works could cause sever financial loss, i would recommend that you patent your dll before you release it...

to answer your question, any executable code can be opend in such a way as to make it human-readable... thing is, only certain humans can read it.

basically the person who wants to reverse engineer will have to have a good working knowledge of assembler or x86 instruction code. (if the computer can understand it, a person can understand it.)

an old trick is to compress or encrypt the dll and have your program decompress or decrypt it.