macOS Preview can add password protection to PDFs, but only when saving a duplicate — not the original. Here is a simpler, more reliable method.
Navigate to pdfeditor.onl/protect-pdf in Safari.
Upload your PDF. Enter a strong password. RC4-128-bit encryption is applied. Download the protected PDF.
Tip: Preview on Mac uses 128-bit RC4 — the same encryption used here. For stronger AES-256 encryption, use Adobe Acrobat or a dedicated encryption tool.
Both use RC4-128 encryption. This tool is simpler — upload, set password, download. Preview requires printing to PDF with a password, which can alter layout.
Yes — completely free in Safari on Mac.