Protect PDF with Password

How the protection works

The tool uses the standard PDF password protection with AES-256, the method that the PDF 2.0 standard recommends. A random 256-bit key encrypts the page content, images, fonts, attachments and document properties such as the title and author, and that key is itself locked with your password. Without the open password, a PDF reader cannot show the pages.

Everything happens in your browser with pdf-lib, a JavaScript PDF library. Before the download is offered, the tool opens the new file with your password, as a reader would, and checks that every page is there. Neither the file nor the password is uploaded.

Open password and owner password

• The open password is needed to open the file. This is the part that actually protects the content.
• The owner password is optional. It lets you restrict printing, copying, editing, form filling and comments, and it is needed later to change those permissions. It must be different from the open password; if both were the same, everyone who can open the file would also have full control.

Permissions are requests, not locks. Adobe Acrobat and many other PDF readers follow them, but some ignore them, and anyone who can open the file can use software that removes them. Use them to discourage casual copying, not to keep content secret. Access for screen readers is always allowed.

Choosing a password

Anyone who has a copy of the file can try passwords on their own computer, as fast as it allows. The encryption is not the weak point; a short or common password is. Length matters more than symbols: three or four unrelated words are easy to remember and hard to guess. Avoid names, dates of birth, mobile numbers and PAN or Aadhaar numbers. The strength meter uses zxcvbn, which recognises common passwords, names, dates and keyboard patterns.

Letters from other scripts, such as Devanagari, work in current PDF apps. Adobe Acrobat changes a few characters before it checks a password, for example full-width letters and invisible joining marks, so the tool warns you when a password contains them.

There is no recovery: if the password is forgotten, nobody can open the file, including us. Keep it in a password manager, and send it separately from the file, for example by phone or text message rather than in the same email.

What changes and what stays visible

The pages are not redrawn, so text, images and links look the same. A few things change:

• Without the password, anyone can still see the file size and some layout details, such as the number of pages, the page sizes and the dimensions of images.
• A digital signature in the original is no longer valid in the protected copy.
• PDF/A forbids encryption, so a PDF/A file loses that label.

Most PDF readers in use today open these files. This AES-256 method needs Adobe Acrobat or Reader X (2010) or later, so very old readers such as Adobe Reader 9 cannot open them.