There are multiple ways to do this. But need to understand your requirements first like what would be mode of transfer of files.
It is impossible to stop sharing of files.
But we can create a pull based solutiion rather than seding files, where your customer's will pull a file from a web server (rather than sending them via email). We can create a mechanism where the files can be pulled only specific machines. The document can be kept accessible via web only, so that it cannot be downloaded (and subsequently shared).
I work in the field of APIs and can suggest an API based solution after understanding your requirements.
It can also be achieved via a mobile application based approach but I am no expert in it.