Locked PDF won't open for the other person?

The quick checklist

A file locked with Protect PDF uses AES 256-bit, and only the password unlocks it. If the password is typed exactly, the file opens. So when it doesn't open, the cause is nearly always about how the password is typed, which app opens the file, or what the website accepts. Check these, in this order:

1. Capital letters, Caps Lock and the keyboard language. The password is case sensitive.
2. An invisible difference: a space at the end, or a phone keyboard that changed a letter.
3. A password in another script that the other person can't easily type.
4. A very old PDF app that doesn't support the lock.
5. A preview instead of a real PDF viewer.
6. A website that can't read locked PDFs.
7. A different file: the original, or a copy with another password.

The good news: your original file is unchanged, because Protect PDF makes a new copy that ends in -protected. You can protect the original again with a password that is easier to type, and send that.

Test before you send, and most of this disappears

Before you send a protected file, open it once yourself:

• Open the new file, the one whose name ends in -protected, in a PDF app or a browser. It should ask for the password.
• Type the password the way you will tell it to the other person, not by copying it from where you made it. A password that you pasted may hide a difference you won't see when you type it.
• Check that it opens, and that every page is there.
• If you can, open it a second way: on your phone and on a computer, or in two different apps.

In our tests, every protected copy asked for its password, refused no password and a wrong one, and opened with the right one, in the PDF engine inside Chrome and Edge (PDFium), in pdf.js, the reader inside Firefox, in Poppler, in pypdf and in qpdf. The lock works in the readers people use. What goes wrong is usually around it.

Cause 1: capitals, Caps Lock and typing slips

A PDF password is case sensitive: Mango and mango are different passwords. These are the usual slips:

• The first letter becomes a capital. On a phone, the keyboard may start a word with a capital letter, or change a word as you type. If you typed the password on a phone and it came out with a capital that you didn't notice, the other person types the plain version and it fails. Use the eye button in Protect PDF to see exactly what you typed before you press the button.
• Caps Lock is on, on the receiver's side.
• Similar-looking characters. A capital I, a small l and the digit 1 look alike in many fonts. A capital O and zero are alike too. Avoid them, or send the password so that the difference is clear.
• A space at the start or the end, which can come along when a password is copied from a message.
• The wrong keyboard language, so the same keys type other letters.

The fix: protect the original again with a password that is easy to type and to say. Three or four unrelated words in small letters, like mango kettle river seven, are easy to remember, type and read out, and they are long enough to be hard to guess. The post How to password protect a PDF for free says more about choosing one.

Cause 2: a password in Hindi or another script

Protect PDF accepts passwords in any language, as AES 256-bit stores them as Unicode text. In our tests, files protected with passwords in Hindi, Japanese and German opened with those passwords in every reader we tried. But the person opening the file needs to type the same letters. If they don't have that keyboard on their phone, they can't. A password in another script is only a good idea if they have that keyboard.

A few characters are refused when you type them in Protect PDF, with a message saying why: emoji, invisible characters, tabs, special spaces, full-width letters and some mixes of right-to-left and left-to-right letters. PDF apps would read them in different ways, and the file might not open everywhere. If you saw such a message and removed the character, make sure you tell the receiver the password without it.

Cause 3: a very old PDF app

PDF files have been locked in several ways over the years: RC4 with 40-bit and 128-bit keys in older files, AES 128-bit, and AES 256-bit, the newest. Protect PDF always uses AES 256-bit, in the form the current PDF standard describes. AES 256-bit has been part of the standard for years, so up-to-date apps open it. But very old PDF apps, from before AES 256-bit came to PDF, can't open it.

The fix: ask the receiver to open the file in an up-to-date web browser, such as Chrome, Edge or Firefox, or to update their PDF app. A browser is the quickest, because it needs no install.

Cause 4: they are looking at a preview

Many email and chat apps show a small preview of a file instead of opening it. A preview of a locked PDF may show a blank page, a lock picture or an error, because it can't ask for a password. That does not mean the file is damaged.

The fix: tell the receiver to save or download the file, and then open it with a PDF viewer or a browser. The password prompt appears there. If you are sending from a phone, the file goes best as an attachment, not as pasted text or a picture of a page.

Cause 5: a website or an office can't read locked PDFs

Some upload portals and offices can't accept password-protected PDFs at all. The site has to read the file to check it, and it can't without the password. If a locked file was turned away, or a form says the file is "corrupt" or "unreadable", this is the likely reason.

The fix: for an upload that must be readable by the website, send the unlocked original and rely on the website's own security. If you only have the locked copy and you know its password, Unlock PDF makes a copy that opens without it. See Remove password from a bank statement PDF for the steps. Unlock PDF removes a password only when you type it; it never guesses one.

Cause 6: the file was protected twice, or with another password

It is easy to mix up files. If you protected the same document more than once, with different passwords, each copy has its own password. The copy you sent may not be the one you tested. Protect PDF names each copy with -protected at the end, and downloads from the same page can pile up in your Downloads folder with similar names.

Another case: you protected a file that was already locked. The page says so on its row and shows a box for the file's current password. You type the current password and the new one, and the old lock is taken off with the current password and replaced by the new one. If the current password is wrong, the row says "That current password didn't open it" and nothing is made. If you meant to just remove a password, Unlock PDF is the tool for that.

The fix: keep the original in one place, delete the test copies, and protect once, with one password. Name the final file clearly before you send it.

Cause 7: the password was sent wrongly

A locked file is only as private as its password, and the way you pass the password on matters as much as the lock:

• Send the password another way. Not in the same email or chat as the file. Tell it in a phone call, a text message or in person, or send it in a different app.
• Read it out slowly, if you say it on a call, and say which letters are capitals.
• Don't put the password in the file name or in the subject line.
• Agree on a password ahead of time with people you send files to often, such as your accountant.

If the receiver says the password is wrong, don't send a second password for the same file: it can't change. The password is the only key to the protected copy, and it isn't stored in the file or by us. Protect the original again with a new password and send the new copy.

If you forget the password yourself

There is no way back. A forgotten password can't be reset, recovered or looked up by anyone, and a good one can't be guessed either. That is the point of the lock, but it works against you if the password is lost. So keep the original, which Protect PDF leaves untouched, write the password down somewhere safe, such as a password manager, and open the protected copy once before you delete anything.

Our tool for removing locks, Unlock PDF, needs the password too. It never guesses one.

The file and the password stay on your device

With Protect PDF, the file and the password never leave your phone or computer. The tool has no upload step and never sends your files or passwords anywhere. The page contacts only this site and Google's ad service, which shows the ads that keep the tools free, and our code never gives your files to the ad code. The password is forgotten when the page closes: it is never saved, logged or sent. The privacy policy has the details.