Hey, lets be reasonable. If we password protect it, it's probably as safe as mailing the client a paper copy is. They can be stolen too.
Odds are, unless your client is extremely rich or famous, the chances of their pdf being hacked are not likely. Yes, it's possible, but it involves the hacker first getting access to the pdf file. Not likely for ordinary folk, unless it's an inside job. And WE should not have to feel responsible for that.