Protect PDF

Protecting a PDF encrypts its contents. Every page, image and stream of text inside the file is scrambled with a key that is derived from the password you set. A reader that is given the right password can rebuild the key and show the document; without it, the file is just unreadable data. This is real protection, not a setting that a different viewer could ignore.

The PDF format supports two passwords, and the tool offers both. The Open password is the one most people want: nobody can see the document without it. The optional Permissions password controls what someone may do after opening the file, such as printing, copying text or editing. The two must be different, and whoever knows the permissions password can lift the limits.

Encryption covers the document from the moment it leaves your device to the moment the recipient types the password. That is useful for files that travel by email, sit in shared folders or get uploaded to cloud storage, where you cannot control who might see them along the way. Payslips, medical letters, contracts and tax papers are typical examples.

Protection does not change how the document looks. Pages, fonts, images and links stay exactly as they were, and the file size grows only slightly. The difference is the password prompt that appears when someone opens it, and the greyed-out buttons in their reader if you set permissions.

The PDF format has gone through several encryption methods over the years. Early versions used RC4, a stream cipher that is no longer considered safe. Later versions added AES with 128-bit and then 256-bit keys. PDF 2.0, published as ISO 32000-2, keeps AES-256 and marks the older methods as deprecated, which is why AES-256 is the default here.

AES-256 has been supported since Adobe Acrobat 9 and opens in every current PDF reader, including the viewers built into browsers and phones. Choose AES-128 only if you know the file must open in very old software. There is no reason to pick it for a document that will be read on current devices.

Encrypt metadata decides whether the document's title, author and keywords are hidden as well. Leave it on for private files, so even the title does not give the content away. Turn it off when the file goes into an archive or document system that has to read the title for search and indexing without knowing the password.

The permissions are worth a closer look. Low resolution only lets people print, but at a resolution that makes a poor copy if the printout is scanned again, which discourages reproducing a document while still allowing a paper copy for reference. The four editing levels go from no changes at all, through filling in forms, to forms plus comments, up to any change except taking pages out. Pick the lowest level that still lets the recipient do their part, such as Fill in forms only for an application they need to complete and sign.

Blocking copying stops text and images from being selected and pasted into another program. It does not stop assistive technology: screen readers can still read the content aloud, so the file stays accessible to people who rely on them.

Encryption is only as strong as the password behind it. A short password, a word from a dictionary or something easy to guess, such as a birth date, can be found by a computer that simply tries many candidates. Length matters most: NIST's digital identity guidelines ask for at least 15 characters when a password is the only thing protecting an account, and they recommend against forcing arbitrary mixes of symbols. A few unrelated words strung together are long and still easy to type.

Send the password separately from the file. If both travel in the same email, anyone who reads that email has both. A text message, a phone call or a password manager's sharing feature keeps them apart, so a lost message or a misdirected attachment does not expose the document.

Keep your own copy of the password somewhere safe. There is no reset link for a PDF: once a file is encrypted with a password nobody remembers, nobody can open it, including you. If the document matters, keep the unprotected original in a secure place too, rather than only the encrypted copy.

Treat permissions limits as a courtesy rather than a lock. They are respected by mainstream readers, but they do not encrypt anything beyond what the open password protects, and a file without an open password can be read by anyone who has it. For confidential material, always set an open password.

Recipients do not need special software to open a protected PDF. The reader they already use asks for the password, checks it, and shows the file; the encryption method is recorded inside the PDF, so they do not need to know which one you chose.

Protect the file last. Once a PDF is encrypted, most tools can no longer change it, so finish compressing, merging, splitting or rotating first. A common order is to merge the pages into one document, compress it for email, and then add the password.

If the document contains form fields or comments that should not change, flatten it first with Flatten PDF. Flattening turns the filled-in values and notes into ordinary page content, and protection then keeps the whole file private. Setting the editing permission to Not allowed adds a further request to readers not to modify it.

Every file in a batch receives the same passwords and settings. That suits a set of documents for one recipient, such as a month of statements for an accountant. When each recipient should have their own password, protect their files in separate batches so that one password never opens another person's documents.

Removing protection later is just as simple, provided you know the password: the Unlock PDF tool makes an unencrypted copy so you can edit or share the document openly again.