however I still don t know very much about the structure of a file (of any extension: txt, doc, exe, pdf, )
You don't need to know what kind of a file it is. All files are just binary data, so if you know how to send and receive data, you can also send a file.

Could you please tell me how can I split a file in its more elemental parts to send it as a String of characters?
You can't send or receive binary data with a CString, because a CString is text.