If you are sending sensitive information like financial records or medical history, simply emailing a PDF is not secure. You need strong encryption.
PDF Encryption Standards
Modern PDFs use AES-256 encryption, the same standard used by banks and governments. When you set a password, the contents of the PDF become completely unreadable without it.
The Offline Advantage
Never use an online server-based tool to add a password to a sensitive document. Instead, use a client-side Protect PDF tool.
This ensures that the encryption happens locally using Web Crypto API, and the unencrypted document is never exposed over the network.