How is this different from other online PDF tools?

Most online PDF tools send your files to a remote server for processing. This tool processes everything locally in your browser using client-side JavaScript. Your files are never transmitted to any server, which means your document content stays entirely under your control.

Are my PDF files uploaded?

No. Your files are read directly by your browser. Your files are never transmitted over the network. The tool works entirely within the browser tab you have open.

Can I verify that files are not uploaded?

Yes. Open your browser's developer tools (F12), go to the Network tab, and watch for any outgoing requests while using the tool. You will see no file data leave your browser. The site may record an anonymous page-view count, which never includes your files and can be disabled.

Does this tool work offline?

Yes, once the page has loaded the tool processes your files entirely in your browser; tool functionality needs no network connection. You can disconnect from the internet and continue using it.

What happens when I refresh the page?

Because nothing is stored on a server, refreshing the page clears your current session. Any files you had selected will need to be selected again.

Does this tool store my files?

No. Files are held in browser memory only for as long as the page is open. Closing or refreshing the page discards them. Nothing is written to disk by the tool, and nothing is sent to a server.

What technologies does this tool use?

This tool uses MuPDF, a PDF engine compiled to WebAssembly, running entirely in your browser to apply AES-256 encryption — your files never leave your browser.

Does this tool use WebAssembly?

Yes — this tool does. MuPDF is C compiled to WebAssembly and runs inside your browser; the .wasm binary is served from this site and neither the file nor the password is sent anywhere.

Can this tool unlock password-protected PDFs?

This page adds a password. To remove one you know, use the companion Unlock PDF tool — it decrypts locally in the same way.

How strong is the encryption?

The document is encrypted with AES-256, the strongest encryption the PDF standard supports and the same algorithm used by Adobe Acrobat's highest setting.

How do I add a password to my PDF?

Pick a single PDF with the file picker, type the password you want and repeat it in the confirm field, then select Protect. The encrypted copy is built in your browser and you save it with the download button.

Can I protect a PDF that already has a password?

No. If the file is already encrypted this tool stops with a message that the PDF is already password-protected, because adding a second layer to a locked document is not supported. Remove the existing password with the companion Unlock tool first, then protect it again.

Are there any restrictions on the password I can choose?

The password can be up to 127 characters and may not contain a comma, because the encryption engine treats commas as option separators internally. The same password you enter is used both to open the document and to change its permissions, and there is no way to recover it if forgotten.

Does the Protect PDF (Add Password) tool cost anything to use?

Password-protecting a PDF here is completely free — no account, no sign-up, and no limit on how many files you encrypt. The tool runs AES-256 encryption directly inside your browser using a WebAssembly build of MuPDF, so your file and your chosen password never leave your device, and it always will be free to use.

Password-Protect a PDF Privately — Encrypt in Your Browser

Select a PDF and choose a password; the file is encrypted with AES-256 locally by MuPDF (compiled to WebAssembly) and the protected PDF downloads from your browser.

Quick answer

What it does: Select a PDF and choose a password; the file is encrypted with AES-256 locally by MuPDF (compiled to WebAssembly) and the protected PDF downloads from your browser.
Free?: Yes — 100% free, no account or sign-up.
Files uploaded?: No — everything is processed in your browser.
Works offline?: Yes — once the page has loaded.
Best for: A consultant encrypts a confidential proposal before emailing it, sharing the password separately with the client.
Good to know: If you forget the password there is no recovery — AES-256 encryption cannot be bypassed.

How it works

Select a PDF file from your device using the file picker.
The file is read into browser memory — no data leaves your browser.
Enter the password you want (it is never transmitted anywhere).
MuPDF — compiled to WebAssembly and served from this site — encrypts the document with AES-256.
The protected PDF is offered as a download; opening it now requires your password.

When to use this tool

A consultant encrypts a confidential proposal before emailing it, sharing the password separately with the client.
An HR manager password-protects a payroll PDF before sending it so only the intended recipient can open it.
A researcher locks a draft paper with a password before sharing it for peer review, limiting unauthorised access.
If you forget the password there is no recovery — AES-256 cannot be bypassed and the file processes locally, so nobody can reset it.

Frequently asked questions

How is this different from other online PDF tools?: Most online PDF tools send your files to a remote server for processing. This tool processes everything locally in your browser using client-side JavaScript. Your files are never transmitted to any server, which means your document content stays entirely under your control.
Are my PDF files uploaded?: No. Your files are read directly by your browser. Your files are never transmitted over the network. The tool works entirely within the browser tab you have open.
Can I verify that files are not uploaded?: Yes. Open your browser's developer tools (F12), go to the Network tab, and watch for any outgoing requests while using the tool. You will see no file data leave your browser. The site may record an anonymous page-view count, which never includes your files and can be disabled.
Does this tool work offline?: Yes, once the page has loaded the tool processes your files entirely in your browser; tool functionality needs no network connection. You can disconnect from the internet and continue using it.
What happens when I refresh the page?: Because nothing is stored on a server, refreshing the page clears your current session. Any files you had selected will need to be selected again.
Does this tool store my files?: No. Files are held in browser memory only for as long as the page is open. Closing or refreshing the page discards them. Nothing is written to disk by the tool, and nothing is sent to a server.
What technologies does this tool use?: This tool uses MuPDF, a PDF engine compiled to WebAssembly, running entirely in your browser to apply AES-256 encryption — your files never leave your browser.
Does this tool use WebAssembly?: Yes — this tool does. MuPDF is C compiled to WebAssembly and runs inside your browser; the .wasm binary is served from this site and neither the file nor the password is sent anywhere.
Can this tool unlock password-protected PDFs?: This page adds a password. To remove one you know, use the companion Unlock PDF tool — it decrypts locally in the same way.
How strong is the encryption?: The document is encrypted with AES-256, the strongest encryption the PDF standard supports and the same algorithm used by Adobe Acrobat's highest setting.
How do I add a password to my PDF?: Pick a single PDF with the file picker, type the password you want and repeat it in the confirm field, then select Protect. The encrypted copy is built in your browser and you save it with the download button.
Can I protect a PDF that already has a password?: No. If the file is already encrypted this tool stops with a message that the PDF is already password-protected, because adding a second layer to a locked document is not supported. Remove the existing password with the companion Unlock tool first, then protect it again.
Are there any restrictions on the password I can choose?: The password can be up to 127 characters and may not contain a comma, because the encryption engine treats commas as option separators internally. The same password you enter is used both to open the document and to change its permissions, and there is no way to recover it if forgotten.
Does the Protect PDF (Add Password) tool cost anything to use?: Password-protecting a PDF here is completely free — no account, no sign-up, and no limit on how many files you encrypt. The tool runs AES-256 encryption directly inside your browser using a WebAssembly build of MuPDF, so your file and your chosen password never leave your device, and it always will be free to use.