What specifically do you mean? Do you want to compress the file in ACS storage, compress the file during the download transport (HTTP compression) or compress the file once it has been downloaded on the client?
That feature depends on how you provide access to your Alfresco system. If you use Apache HTTPs as a proxy, check mod_deflate. Tomcat itself also supports compression (look for compressibleMimetype in Apache Tomcat 8 Configuration Reference (8.5.37) - The HTTP Connector ). Either way, you will have to define which mimetypes the server / proxy should compress when sending to the client. For some mimetypes it will not make much sense to use HTTP compression as they are already naturally compressed, and the gains would be very minimal at best.
