Send non-editable files when you are asking others to review your work. This way the files can be marked up and sent back to you.
Examples include:
JPEG, PNG – these files can be marked up using screenshot software.
PDF – this can be marked up using any markup software.
If a master file is collaborated on by more than one person then it should be managed via a central location, e.g. Dropbox. Do not email source files backwards and forwards as management of those files becomes difficult.