If you've visited the Send Files Securely website, then you are already aware that email is not secure (e.g. Are email attachments secure?). Steve Gibson also explained this on Security Now Episode 408 (see video clip below).
As an alternative to sending private files over email, you can use Send Files Securely. All files uploaded will be encrypted and by adding a password you can ensure that only those you authorize can download and view the file.This solves the problems Steve is referring to in this video clip.