How to password protect a PDF with AES-256 encryption
Updated · PDF Converter
Payslips, contracts, tax returns and medical letters often travel by email or sit in shared folders, where they can be forwarded to the wrong person. Adding a password means that anyone who gets hold of the file still cannot open it without the password. This guide shows how to encrypt a PDF for free, which settings matter, and how to take the password off again.
The quick way: add an open password
- Open Protect PDF and add your PDF. You can add several files; each one is encrypted with the same password.
- Type a strong password in Password to open the PDF. This field is required.
- Choose the permissions (see below), or leave the defaults.
- Press Protect PDF and download the encrypted file.
Open the downloaded file to check: your PDF reader should now ask for the password before showing anything.
How strong is the protection?
The file is encrypted with AES-256 using QPDF – the strongest standard encryption available in the PDF format. Without the password, the content of the file cannot be read. In practice, the weak point is the password, not the encryption: a short or common password can be guessed, so use at least 12 characters, ideally a random mix or a long passphrase.
AES-256 PDFs open in readers from roughly 2008 onwards – Acrobat 9 and newer, all modern browsers, and the PDF apps on current phones and computers.
Permissions: printing, copying and editing
Besides the open password, you can limit what people may do once the file is open:
| Option | Default | What it controls |
|---|---|---|
| Allow printing | On | Whether the reader lets people print |
| Allow copying text and images | On | Whether text and pictures can be copied out |
| Allow editing | Off | Whether the document may be changed |
These restrictions are stored with a second password, the Permissions password. It is optional: if you leave it empty, a random one is used. If you later want an unrestricted copy yourself, Unlock PDF can remove the restrictions as long as you know the open password.
Be realistic about permissions. Mainstream readers honour them, but they cannot technically stop a determined person once the file is open. The open password is the real protection.
Tip: send the password through a different channel from the file – for example, email the PDF and share the password by phone or text message. If both travel together, the protection is only as safe as that one inbox.
Before you encrypt: remove what should not be there
A password keeps strangers out, but the people you send it to see everything inside. If parts of the document should not reach them at all, deal with that first:
- Use Redact PDF to permanently remove account numbers, names or other sensitive passages.
- Use Sanitize PDF to strip hidden extras such as embedded JavaScript, attachments, metadata and comments.
Then protect the cleaned file.
How to remove the password later
If you get a password-protected PDF – a bank statement, for example – and are tired of typing the password every time, use Unlock PDF. Enter the current password and download a copy that opens without it and has no printing or copying restrictions. Files that open normally but block printing can usually be unlocked with the password field left empty.
Unlock PDF does not crack or guess passwords. For files that need a password to open, you must know it.
Is my password safe?
Yes. Encryption runs inside your browser. Neither the document nor the password is uploaded, and PDF Converter never sees or stores your password.
Frequently asked questions
How do I password protect a PDF for free?
Add the PDF to Protect PDF, type a password, choose the permissions and download the encrypted file. The encryption runs in your browser, so there is nothing to install.
What if I forget the password?
There is no way to recover it. AES-256 is designed so that the file cannot be opened without the password, so keep an unencrypted copy somewhere safe.
Can I password protect a PDF on my phone?
Yes. Open the page in your mobile browser, choose the PDF from your files, enter a password and download the protected copy.