I know it's impossible to reverse a dll into a c++ code so I would like to collect as much as possible details from it. It's not my dll, so I don't have the source code of course. Which program should I use?
Well, if you are skilled you can disassemble the DLL and understand all of its functions. This takes a substantial amount of time, but if you do that you can reverse it back to source by hand.
Otherwise, you can start by using a tool like Dependency Walker to get the DLLs and functions it depends on, and the functions it exports. From there you can find functions that interest you, and use a disassembler like IDA to see what they do.