Email is convenient, but it is not a private filing cabinet. Attachments can be forwarded, synced, backed up, and opened on shared devices. If a PDF contains personal or business-sensitive information, adding a password is a sensible extra step.
Use a password that is not obvious, and send it through a different channel from the PDF. For example, email the protected PDF and send the password by phone or chat. Do not put both in the same email thread.
Protect PDF applies encryption to the document so the recipient needs the password to open it.