Protect PDF
Add an open password (AES-256) to a PDF and save it. Files never leave your device.
No upload · Processed in your browserPDF200 MB per file (recommended)As of 2026.10.01
Locks with AES-256 and allows printing, copying, and editing once opened.
If you forget this password, the file can't be opened. The password isn't saved anywhere.
How to use Protect PDF
- 1Drag and drop one PDF that has no password, or pick it with [Choose files]
- 2Enter the same password in Open password and Confirm password, then press [Start]
- 3Save the locked PDF with [Download]. Keep the password somewhere safe
The big number on the result card is the number of pages. The result keeps the original name with -protected added, as in contract-protected.pdf, and asks for the password when opened. You can unlock it again in Unlock PDF with the same password.
You can add one PDF at a time. We recommend up to 200 MB on desktop and 50 MB on mobile. Going over the recommendation shows a warning, but you can still continue. Files over 500 MB are not processed.
- Example 1 · Add an open password
a-3p.pdf (3 pages, 2 KB) · same password entered in Open password and Confirm password
Measured in Chrome on desktop · test PDF with one large label per page
→ a-3p-protected.pdf, 3 pages · 2 KB · AES-256 (the PDF 2.0 method) · won't open without the password
- Example 2 · Unlock it again with the same password
a-3p-protected.pdf from Example 1 · added to Unlock PDF with the same password
Measured in Chrome on desktop · test PDF with one large label per page
→ a-3p-protected-unlocked.pdf, 3 pages · page labels unchanged
- Example 3 · When the confirmation doesn't match
a-3p.pdf · Open password abc, Confirm password abd
Measured in Chrome on desktop · test PDF with one large label per page
→ Short-password warning · “Passwords don't match” · [Start] can't be pressed · changing the confirmation to abc enables [Start]
Example figures vary by file and device.
Protect PDF options explained
- Open password
- The password asked for when the PDF is opened. Any length of 1 character or more works, but anything shorter than 6 characters shows the warning “Short passwords are easy to guess”. You can still continue, but unless you've agreed on a rule with the recipient, use a long password that's hard to guess.
- Confirm password
- Type the same password again. If the two fields differ, “Passwords don't match” appears and [Start] can't be pressed. This keeps a typo from creating a file nobody can open.
- Show
- The password you type is hidden at first. Press [Show] to see the characters, and press it again to hide them. The fields are set so the browser's autofill doesn't remember them.
- How the file is locked
- This isn't a setting you can turn on or off; it always works the same way. The file is encrypted with AES-256 (the PDF 2.0 method), and the permissions password is set to the same value as the open password, so printing, copying and editing are all allowed once the file is open. It doesn't set print-only or copy-only restrictions. Page content stays as it is.
Protect PDF: frequently asked questions
Are my PDFs and passwords sent to a server?
No. Adding a password happens inside this browser, and your files and the password you enter are never sent off your device. The password isn't saved anywhere. To check it yourself, follow the steps on the How it works page.
What if I forget the password?
You won't be able to open the file. ZEKILO Studio doesn't save the password, so there's no way for us to look it up either. If you might need it, keep the original without a password somewhere safe, and write the password down where you won't lose it.
How is the file locked?
It's encrypted with AES-256. Among the PDF 2.0 specification's standard encryption methods, it's the only one that isn't deprecated (R6). Very old PDF apps may not support it, so open the file once in the app the recipient uses to check.
Can I block only printing or copying?
This tool sets only an open password. The permissions password is the same as the open password, so anyone who opens the file with the password can print, copy and edit it.
Can I add a new password to a PDF that already has one?
Not directly. Adding a password-protected PDF shows “Password-protected PDFs can't be processed” and points you to Unlock PDF. Unlock it first with the password you know, then add it again and set a new password.
Does adding a password change the file size?
Encryption information is added, so the size can change a little. The 3-page test file still showed as 2 KB after locking. Page content isn't redrawn, so quality stays the same.
Related tools
How it works · Engine · Notes
- Processed in: this browser (your device)
- Engine: qpdf (WebAssembly build) (Apache-2.0) · qpdf-wasm (@neslinesli93/qpdf-wasm, wrapper) (ISC) · PDF.js (file list thumbnail) (Apache-2.0)
- Adding a password runs inside this browser with qpdf, and files and passwords are not sent to a server.
- The file is encrypted with AES-256 (the PDF 2.0 method). The permissions password matches the open password, so printing, copying and editing are all allowed once the file is open.
- The password you enter isn't saved or included in analytics or logs. If you forget it, the file can't be opened.
- The qpdf engine (WebAssembly, about 1.3 MB) is downloaded once, the first time you add a PDF.
- The first-page thumbnail in the file list is drawn with PDF.js for display only and doesn't affect the result file.
- As of 2026.10.01
- Changelog: 2026.10.01 First release