The confidentiality of the materials you create for you client is very important. And, the bigger the account the bigger the stakes. Email is not a secure communication channel by default. Admins of every server that your email passes through can theoretically read your non-encrypted email.
Therefore, it doesn't hurt to add an extra layer of security when sending through important documents and presentations to your client. So, how to secure your documents without complicated additional software?
Securing your pdfs and jpegs is very easy. You can open them in Preview and resave them as pdf with the Encrypt option switched on.
Now, enter your password twice.
When your client or any other unintended recipient tries to open the pdf it will ask for a password.
Make sure to send the password in a separate email or other communication channel, such as IM or phone.
If you are saving from Adobe graphics or Apple's iWork applications you will have extra options to secure your pdf separately securing against copying or printing. Altough I think both of these options are pretty much useless, because they don't protect against retyping or a screen-shoot.