How to password-protect a PDF, and when restrictions are enough

An open password is the real protection

When you set an open password, the PDF's content is encrypted with a key derived from it. A viewer asks for the password before it shows a single page, and there is no other way in: not for the recipient, not for Toolkitta, not for anyone who intercepts the file. Protect PDF uses AES-256 with the current security handler (revision 6), the method ISO 32000-2 specifies and Adobe Acrobat has used by default since version 9.

The strength of the encryption is fixed; the strength of the protection is your password. Password-recovery software tries millions of short passwords per second, so a birth date or a six-character word is a formality. A sentence you can remember — four or five unrelated words — is both easier to type and far beyond such attacks. There is no recovery: if you forget the password, the file is gone for good, which is why you keep the unprotected original.

Adding a password

1. Open Protect PDF and add the file, or up to 20 files that should share the same password. Each row shows the page count and a preview.
2. Leave Protection on Password to open. Type the password and repeat it; the button stays disabled until both match, so a typo cannot lock you out of your own file.
3. Optionally open Also restrict what others can do and tick printing, copying, editing or comments. These apply even to someone who has the password.
4. Click Protect and download the copy, named after the original with -protected added. The copy is reopened with your password before it is offered, so a file that failed that check would be reported rather than handed over.

Encryption runs in your browser. The file and the password never leave your device, the password is not stored, and the original file on disk is not modified. Then share the password by a different route than the file: a message on another app, a phone call, in person. A password in the same email as the attachment protects against nothing.

Restrictions only: useful, but not a lock

Sometimes the document should be readable by anyone but not changed: a price list that must not be edited, a proof that should not be printed, a form to fill in but not restructure. Switch Protection to Restrictions only, tick what to block, and the file opens without a password while Acrobat, Preview, browsers and phone viewers disable the ticked actions.

Be clear about what this is. A restriction-only file is encrypted with an empty open password, so any PDF library can decrypt it, and whether the permission flags are honoured is the reader's choice. Toolkitta's own Unlock PDF lifts them without asking for anything, and so do many desktop tools. Restrictions prevent accidents and state your intent; they do not keep content from someone determined. If confidentiality matters, set an open password — with or without restrictions on top.

• Printing: the print command is disabled, including print-to-PDF in most viewers.
• Copying text and images: selection and copy are disabled; accessibility readers are still allowed to extract text, and screenshots are naturally unaffected.
• Editing pages and content: inserting, deleting, rotating and altering pages is disabled.
• Adding comments and filling forms: annotations and form entries are disabled; tick this only if the form is meant to be read, not completed.

Which viewers open the protected file

AES-256 has been supported since Adobe Acrobat and Reader 9 in 2008, and is read by the PDF viewers built into Chrome, Edge, Firefox and Safari, by macOS Preview, by the Windows and Android viewers and on iOS. Software older than that used the weaker RC4 method, which Protect PDF deliberately does not offer: it is broken, and the recipients who would need it are vanishingly rare. If a recipient reports that the file will not open, the cause is almost always a mistyped password or an ancient viewer; ask them to try the browser.

Inside the file, nothing else changes. Pages are not copied or flattened, so bookmarks, links, comments, editable form fields and the document's metadata are kept — the metadata is encrypted with the rest, so the title and author are not visible without the password either. File size grows by a few hundred bytes.

What Protect PDF will not do

It refuses files that already carry a password or restrictions, because layering encryption over encryption is never what anyone wants; remove the old protection with Unlock PDF first, then protect with a new password. It refuses digitally signed documents, since re-saving would invalidate the signature, and XFA forms, which cannot be rewritten faithfully in the browser. It does not redact: a password protects the whole file, it does not remove anything from it. If parts of a document must not be seen even by the recipient, remove them before protecting.

Finally, it cannot help you recover a password you set and forgot. That is not a missing feature; it is what makes the protection real. Keep the unprotected original where you keep your other important files, and treat the protected copy as the one that travels.