How to send documents securely: what email itself offers
Mail between major providers is encrypted in transit, which protects it on the wire and nowhere else. Gmail's confidential mode can set an expiry and block forwarding and downloading, and Outlook can encrypt a message with Microsoft 365 Message Encryption; both keep the attachment in the mailbox for the life of the message and work best when both sides use the same provider. A password-protected PDF or ZIP helps too, as long as the password does not travel in the same email. The Gmail attachment page covers those options step by step.