Password protect a PDF
Lock a PDF with AES-256 before you email it. Encrypted in your browser; the file and the password stay on your device.
Open the PDF you want to lock.
This PDF already has password protection. To change its password, first remove the password with Unlock PDF, then open the unlocked copy here.
This PDF has a signature field. If it is digitally signed, the signature will no longer verify after protecting, because the file is rewritten. The signature's picture stays on the page.
Your PDF and password stay on this device. Both are used inside this page and nowhere else. There is no upload and no account; once the page has loaded it keeps working in airplane mode.
How to password protect a PDF here
- Open the PDF. Tap Choose a PDF, drop the file on the box, or paste it. A file that already has a password is sent to Unlock PDF first.
- Type the open password twice. Tap Show to check it. The line under the box gives a rough strength guide from the length, the mix of characters and a list of the most-used passwords.
- Choose the options, or leave the defaults. AES-256 suits every current reader. Add a permissions password only if you want readers asked not to print or copy, and choose whether to keep the title and author.
- Tap Protect PDF. The file is encrypted here, then opened again in PDF.js with your password: the page count and a picture of page 1 appear, and a wrong password is tried and must fail.
- Save it and send it. The copy is named after the original with -protected added. Send the password by text message or phone, not in the same email as the file.
What it cannot do
- It cannot recover or reset a password. Not yours, not anyone's. If the password is lost, the protected copy is lost; keep the original.
- Permission locks are honoured by readers, not enforced by the encryption. Anyone who has the open password can decrypt the whole file, and a program that ignores the flags can then print or copy it.
- Anyone who opens the file can copy what they see. A screenshot or a photo of the screen is always possible. The password decides who can open the file, not what they do after.
- A digitally signed PDF loses its signature's validity. Encrypting rewrites the file, so a signature made over the old bytes shows as changed. The page warns you when it finds a signature field.
- It does not change a password on an already protected PDF. Remove the old one with Unlock PDF first.
What gets encrypted, and how
A PDF is a set of numbered objects: page drawings, fonts, images and attachments are streams; the title, author, bookmark names, form field names and the values typed into fields are strings. Password protection works only if every one of them is encrypted. This page encrypts each string and each stream with its own random 16-byte starting value, using the key that the password unlocks, and writes the /Encrypt dictionary that tells a reader how. Only that dictionary and the file identifier stay readable, as the standard requires.
That is checked, not assumed. One popular open-source PDF library's built-in encryption, run on a test file and saved without object streams, left the document title and a form value readable in the bytes; this tool does not use it. Its test builds a one-page file with a title, a filled form field and an attached receipt, protects it, and searches the raw bytes and every decompressed stream for each of those words: none is found.
AES-256 is the PDF 2.0 method (/V 5 /R 6). The file is encrypted with a random 256-bit key; your password only unlocks that key, through a hash that runs at least 64 rounds of AES and SHA-2 per attempt, so every guess costs an attacker real work. AES-128 (/V 4 /R 4, PDF 1.6) is for readers from before about 2010; its key comes from 51 rounds of MD5, which is far quicker to guess against.
The password still matters more than the cipher. A random 12-character password of lower-case letters and digits has about 62 bits, which is out of reach of guessing; Summer2026! is 11 characters but built from a word, a year and a symbol, which is exactly what guessing programs try first. The strength line is a rough guide that counts characters; it cannot tell that a password is your dog's name.
Under the hood, @cantoo/pdf-lib 2.11.1 (MIT) reads and writes the file, the encryption is written in this page's protect-core.js with your browser's Web Crypto, and PDF.js (the viewer built into Firefox) re-opens the result as the check. Both libraries are pinned copies served from this page and load only when you open a file.
Questions
How do I password protect a PDF to send by email?
Open the PDF here, type a password twice and tap Protect PDF. After the check, save the copy (name-protected.pdf) and attach it. Send the password another way, by text message or a call: if it travels in the same email, anyone who can read the email can read the file.
Can I password protect a PDF on an iPhone?
Yes. Open this page in Safari, tap Choose a PDF and pick the file from Files. Type the password twice, tap Protect PDF, then Save protected PDF; Safari puts it in Downloads in the Files app, ready to attach to a mail.
How do I password protect a PDF on Windows without Acrobat?
Windows has no built-in way to add a password to an existing PDF, and Microsoft Print to PDF cannot set one. Open this page in Edge, Chrome or Firefox, drop the PDF on the box, and save the protected copy. Nothing to install, and the same steps work on a Mac, Linux or a Chromebook.
Is it free, and is my file uploaded?
Free, with no account, no watermark and no file-size limit set by us. The file is not uploaded: it is read and encrypted inside your browser, and once the page has loaded it works in airplane mode.
What happens if I forget the password?
Nobody can recover it, this page included: the password is not stored anywhere, and AES-256 has no back door. Keep the original file, and note the password somewhere safe before you send the protected copy.