How to add a password to a PDF

A password-protected PDF requires anyone who opens it to enter the correct password first. The document's contents are encrypted — without the key, the file is mathematically unreadable. It is the most straightforward way to ensure that a payslip emailed to the wrong address, a draft contract shared via a public link, or a scan of an ID document cannot be read by anyone who intercepts or stumbles across the file.

What password protection actually does

When you add an open password to a PDF, the file is encrypted using AES (Advanced Encryption Standard). Any PDF reader — Adobe Acrobat, Apple Preview, Chrome's built-in viewer, or any mobile PDF app — will prompt for the password before revealing any content. Without the correct password, the file cannot be opened. This is not a superficial lock that a simple trick can bypass; the content is cryptographically scrambled and only the right password can unscramble it.

How strong is PDF encryption?

Modern PDF encryption uses 128-bit or 256-bit AES — the same algorithm governments and banks use to protect sensitive data. A well-chosen password makes a brute-force attack computationally impractical: even with fast hardware, exhausting all combinations of a strong password would take longer than a human lifetime. The encryption is only as effective as the password itself, however. Short, common, or predictable passwords can be cracked quickly with freely available dictionary-attack tools.

• Avoid: 'password', 'pdf123', your name, birth year, or any word that appears in the document.
• Prefer: a passphrase of four or more random words (such as 'correct-horse-battery-staple') or a random string generated by a password manager.
• Send the password via a separate channel — for example, a text message — rather than in the same email as the protected file.
• Store the password in a password manager the moment you set it; there is no recovery mechanism.

Step-by-step: adding a password

• Open the Protect PDF tool.
• Upload the PDF you want to encrypt.
• Type a strong password and confirm it in the second field.
• Click 'Protect PDF'. The file is encrypted locally in your browser — nothing is transmitted.
• Download the protected PDF and store the original in a safe, private location.

What happens if you forget the password?

There is no recovery path. Because encryption is designed to make the contents unreadable without the key, neither we nor any tool can retrieve your document if the password is lost. If you misplace the password to a protected PDF and have no unprotected backup, the content is effectively inaccessible. This is why noting the password in a password manager before protecting the file is so important.

When a password is not enough

A PDF password prevents unauthorised opening, but it does not stop the authorised recipient from printing the document and photographing it, forwarding the file together with the password, or taking a screenshot. For documents that require stricter controls — regulated financial records, legally sensitive NDAs, healthcare data — consider a dedicated digital rights management solution in addition to basic password protection, and take legal advice on the appropriate handling requirements.