Encrypt with AES-256 and optionally restrict printing, copying, or editing.
Lock your PDF with a password before sending it anywhere. Files are encrypted with AES-256 — the same standard banks use — and you can additionally block printing, text copying, or editing.
AES-256, the modern standard for PDF encryption. Without the password, the contents can't be read.
Beyond the open password, you can disallow printing, copying text, and editing the document.
There's no backdoor — that's the point of encryption. Store the password safely and keep an unencrypted original.