Protect PDF
Put a password on a PDF so that only people who know it can open it. The file is encrypted with AES-256, the current standard, and you can also withhold printing, copying and editing. Everything happens in your browser: the document and the password never leave your device.
Drop a PDF file here
PDF only, up to 200 MB. One file at a time.
Your files stay in this browser tab. Nothing is uploaded, saved or logged.
How Protect PDF works
The PDF is opened in a background process of your browser and rewritten with its content encrypted by AES-256, using qpdf compiled to run locally. Text, images and fonts are all encrypted, so nothing readable remains in the file: a program that does not have the password cannot display the pages or extract the words.
Two passwords exist in a PDF. The password to open is the one you choose here. The owner password governs the permissions: printing, copying text and images, and editing. If you withhold a permission and give no owner password, one is generated, shown once after the file is made, and not stored anywhere. These limits are respected by the common PDF programs, but they are a request, not a lock: only the password that opens the file truly protects its content.
The password is turned into the encryption key on your device and is not kept: after the file is made, the fields are emptied. This also means it cannot be recovered. If you lose it, the file cannot be opened by PDFStrong or anyone else, so keep the password somewhere safe.
How to use it
- Select a PDF file, or drop it on the area at the top of the tool.
- Type the password and repeat it. Use the Show button to check what you typed.
- Choose what people who open the file may do: print, copy, edit. Add an owner password if you want to be able to change those limits later.
- Choose Protect PDF and download the protected file. Open it once to check that it asks for the password.
Example
| Password | Outcome |
|---|---|
| An empty password (0 bytes) | Enter a password. |
| Correct-Horse-Battery-9 (23 bytes) | Accepted |
| 42 Japanese characters (126 bytes) | Accepted |
| 43 Japanese characters (129 bytes) | The password is too long. The limit is 127 bytes, about 127 letters. |
| 128 letters (128 bytes) | The password is too long. The limit is 127 bytes, about 127 letters. |
Limits and good to know
- The PDF can be up to 200 MB. The document is held in your device’s memory while it is encrypted.
- A PDF that is already protected cannot be protected again: remove its protection with Unlock PDF first, then protect the new copy.
- AES-256 files open in Acrobat X and later, in current browsers and in current PDF programs on computers and phones. Very old readers, from before 2010, may refuse them.
Frequently asked questions
Are my PDF and my password sent anywhere?
No. Both stay in your browser tab. The file is encrypted on your device and nothing is uploaded, which you can check in your browser’s developer tools: the Network tab shows no upload while you protect. The password fields are emptied once the file is made.
What if I forget the password?
The file cannot be opened. The password is not stored or sent anywhere, so PDFStrong has no copy and no way to recover it, and AES-256 cannot be broken in a reasonable time by anyone else either. Keep the password in a password manager, and keep an unprotected copy of the original somewhere safe.
How strong is the protection?
The encryption is AES-256, which is strong. The weak point is the password: a short or common one can be guessed, and a long, random one cannot. Prefer 12 or more characters, such as several unrelated words, and do not reuse a password from another account.
Do the print, copy and edit limits really stop people?
They stop people using the usual PDF programs, which honour them. They are not a security barrier: someone with the password to open the file can have the content on screen, and a program that ignores the limits can use it. Treat them as a courtesy, and rely on the password for real protection.
Can I remove the password later?
Yes, with Unlock PDF, as long as you know the password. It produces a new file without any protection. Only do this for documents that you own or may unlock.