Who says you can't?

I password protect files and attach to email when a client requests it.

What's stopping you?

Print to Email (PDF) in Lacerte password protects it, and there it goes, even fills in the contact information if you had that from client previously.

Print to paper & scan it yourself e-mail it to whomever you desire! 

Are there other forces at work in the Universe preventing you from completing this task?

*when e-mailing personal, financial and any & all other "sensitive" information for goodness sake use a password