RedactNinjaPDF

How to password protect a PDF

An open password on a PDF means the file cannot be opened without it. This guide shows how to add one with RedactNinja PDF, what the protection does, and how to share the password safely.

Worked example: protecting a deposit summary

In this test we protected a fictional one-page tenancy deposit summary of 3,602 bytes. We first typed an eight-character password. The tool showed "Use at least 12 characters for the password." and the download button stayed disabled. With a 23-character passphrase made of three words and a number, Download PDF produced a 3,288-byte file.

We checked the download with qpdf, an independent PDF tool. It reported 256-bit AES encryption (security handler revision 6). Without the password, text extraction was refused; with it, the text came out normally. When we opened the file in RedactNinja PDF again, it asked for the document password first. The download had the same file name as the original, so rename it to avoid mixing them up.

Add a password to your PDF

  1. Open Protect PDF and select or drop the PDF. The Export document dialog opens.
  2. Type your password under Password protection. It must be at least 12 characters.
  3. Leave Format as PDF document. Image exports (PNG or JPG pages in a ZIP) cannot be password protected.
  4. Select Download PDF. Before the download starts, the tool checks that the file is locked without the password and opens with it. If that check fails, nothing is downloaded.
  5. Open the downloaded file in your usual PDF reader and enter the password to make sure it works.

What the password protects

This adds an open password, sometimes called a user password. The whole document is encrypted with 256-bit AES, so the pages cannot be read until someone enters the password.

It does not set permission restrictions. Once the file is open, the reader can print it, copy text and save a copy. Restrictions of that kind depend on PDF software choosing to honour them, which is one of the points covered in what PDF passwords do and do not protect.

Encryption is also not redaction. Everyone who has the password sees the whole document, including any hidden text, comments or attachments. If some of the content should not go to the recipient, remove it first, then protect the copy.

Choose a password that is hard to guess

Strong encryption does not help if the password is easy to guess. The UK National Cyber Security Centre suggests combining three random words to make a password that is long and memorable. Avoid names, dates and words connected with the document, such as the client's surname or the case reference.

A password manager can create and store a long random password for you. Use a different password for each file or each recipient, so that one shared password does not unlock everything you have sent.

Send the password separately

Never put the password in the same email as the file. The ICO advises sending the password by a separate channel, for example by telephone once the recipient confirms the email has arrived. A text message or a call means someone who intercepts the email does not also get the password.

If you forget the password, the file cannot be recovered. RedactNinja does not keep a copy of it. Keep the unprotected original in a safe place, or keep the password in your password manager.

Opening and unlocking a protected PDF

Any standard PDF reader asks for the password when the file is opened. To edit a protected PDF in RedactNinja PDF, open it, enter the Document password, tick I am authorised to edit this document and select Unlock and open. The editor works on a decrypted copy, so set the password again when you export.

To save a copy without a password, use Unlock PDF. It only works with the correct password; it does not bypass or crack one.

Before you send a protected PDF

Encryption happens in your browser on your device. The file and the password are not uploaded.

Related tools and guides

Sources

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