How to password-protect a PDF in your browser

Protect adds an open password and AES-256 encryption to a PDF entirely in your browser — the file and the password never leave your device.

Before you start

• Protect works on PDFs that are not already encrypted. If your file already has a password, unlock it first with its current password, then re-protect it with new settings.
• The encryption engine (qpdf compiled to WebAssembly) runs inside your browser tab. Nothing is uploaded to any server — which also means AttachKit cannot recover a password you forget, so store it somewhere safe.
• The drop zone caps files at 100 MB to keep your browser responsive. For larger files, split them with Pages first.
• Protect is free and doesn't require an account.

Steps

1. Open Protect.
2. Drag your PDF onto Drop a PDF here, or click Choose PDF to pick a file. AttachKit briefly checks the file to confirm it's a readable, unencrypted PDF.
3. Type a password in Password to open the PDF. This field is required — it's the password anyone, including you, will need to open the protected copy.
4. Review the Permissions checkboxes. All four are allowed by default; uncheck anything you want restricted:

1. Click Protect + download. The button shows "Protecting…" while encryption runs, then your browser downloads the protected copy automatically.

How the permissions work

Unchecked items are marked restricted in the PDF. Adobe Acrobat and most desktop readers honour these flags, but many web-based viewers ignore them — treat permissions as a request, not a hard lock. If something must never be read or copied, redact it before protecting.

Two details AttachKit handles for you:

• It generates a random owner password you never see. A PDF's permission flags only bind when the owner password differs from the open password, so this makes your restrictions actually take effect. You keep full control through your open password and can remove the protection at any time with Unlock.
• Text extraction for accessibility is always left allowed, so screen readers keep working even when "Copying text" is unchecked.

Result

You get a copy named after the original plus a -protected suffix (for example contract-protected.pdf), encrypted with AES-256. A confirmation appears: "Protected … — it now needs your password to open." The original file on your device is untouched. Anyone opening the protected copy is prompted for the password, and readers that honour permission flags enforce the print, copy, and edit restrictions you set.

To undo the protection later, open the protected file in Unlock and enter the open password.

Related

• Protect troubleshooting — already-encrypted files, invalid PDFs, and permissions that readers ignore
• How to unlock a PDF — remove a password you know
• How to redact a PDF — permanently remove sensitive content instead of just restricting it