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SECURE
Add a password so only the people you choose can open the file.
QUICK ANSWER
To lock a PDF, add the file here, type a password twice and download the protected copy. StackPDF encrypts it with AES-256, so nobody can open it without the password, in any PDF reader. It all happens in your browser, so the password never leaves your device.
Updated 8 November 2026 · Written by the StackPDF teamUpdated 8 November 2026 · StackPDF team
You’ll choose the password next. It stays in this browser.You’ll choose the password next. It stays in this browser.
QUICK ANSWER
To lock a PDF, add the file here, type a password twice and download the protected copy. StackPDF encrypts it with AES-256, so nobody can open it without the password, in any PDF reader. It all happens in your browser, so the password never leaves your device.
Updated 8 November 2026 · Written by the StackPDF teamUpdated 8 November 2026 · StackPDF team
HOW TO
Drop it on this page, or choose it from your computer, your phone or My files.Choose it from your phone or from My files.
Type it twice. Add limits on printing, copying or editing if you need them.Type it twice. Add print, copy or edit limits if you need them.
Share the file, and send the password separately, for example by text message.
PASSWORD TIPS
Length matters more than symbols. Three or four random words work well.
Pick one you don’t use for email, banking or other files.
Share the file by email and the password by text message or a call.
StackPDF can’t recover a forgotten password. Store it in a password manager.
GOOD TO KNOW
You can set one or both. Use an open password for anything private.
| Password | What it does | Also called | When to use it |
|---|---|---|---|
| Open password | Needed to open the file at all | User password, document open password | Anything private. Always set on StackPDF. |
| Permissions password | Allows or blocks printing, copying and editing | Owner password, restrictions password | To limit what readers can do. Optional. |
Some PDF apps ignore print and copy limits, so don’t rely on them alone for private files.
PRIVACY
Yes. Encryption happens on your device, and the password is never sent to StackPDF.
StackPDF encrypts the PDF inside your browser tab.
It’s used once to encrypt the file, then discarded. We can’t see it or recover it.
The strongest encryption the PDF format supports, applied by qpdf, an open-source PDF library that runs in your browser.The strongest encryption the PDF format supports, applied by the open-source qpdf library.
ANY DEVICE
There’s nothing to install. Open this page in a current browser on your computer or phone.Nothing to install. It works in your browser.
Open stackpdf.com/lock-pdf in Safari, Chrome or Firefox.Safari, Chrome or Firefox
Works in Edge, Chrome and Firefox on Windows 10 and 11.Edge, Chrome or Firefox
Open this page in Safari.Safari
Open this page in Chrome.Chrome
COMPARE
Each one encrypts the file. Here’s how they compare.Each one encrypts the file.
| Method | Cost | Works on | Install |
|---|---|---|---|
| StackPDFThis page | Free for 1 export a day. Premium is $5.99 a month. | Any browser, iPhone and Android | Nothing to install |
| Adobe Acrobat Pro | Paid subscription | Windows and Mac | Desktop app |
| Preview | Free | Mac only | Built into macOS |
FAQ
Add the PDF to this page, type a password twice and download the locked copy. The free plan includes 1 PDF export a day, with no account.
Yes. This page locks the PDF with nothing to install, and the locked file opens in Acrobat and every other PDF reader.
With AES-256 and a long password, nobody can open it without the password. Short or common passwords can be guessed, so use 12 or more characters.
Yes. Turn off printing, copying or editing when you set the password. Some PDF apps ignore these limits, so don’t rely on them alone for private files.
StackPDF can’t recover it, and nobody else can open the file either. Keep the password in a password manager, and keep an unlocked copy somewhere safe.
No. Lock PDF runs in your browser, so the file stays on your device and the password isn’t kept.
Yes. Open this page in Safari and lock the PDF the same way.
Use Remove password from PDF and type the password once. You get a copy that opens without it.