Can I send password-protected documents?

We would recommend considering one of these two methods to protect any sensitive information:

1. A password-protected PDF: you could link from your campaign to a PDF that requires a password to open/view.
2. A password-protected survey: configure the settings of a survey to require a password in order to view or respond to the survey. You could create a one page content-only survey that includes any sensitive information.

Of course with both of these options, you may find that the recipient may share the password to the document or survey. However, clicks on the link and views of the survey will be recorded in reporting data so you may be able to tell if someone had done so.