What does "password-protect a PDF" mean, and what is AES-256 encryption?
Password-protecting a PDF means encrypting the file itself so the document cannot be opened, printed, or copied without the password. The PDF specification supports several encryption algorithms; the modern, industry-standard option is AES-256 (Advanced Encryption Standard with a 256-bit key), which is the same encryption standard used for classified US government documents. ScanLens applies AES-256 encryption on-device before export — the encryption happens on your iPhone, the password is never transmitted, and the protected PDF is ready to email, upload to a portal, or drop in a shared drive without exposing the content.
Password protection is for cases where the PDF itself should not be readable by whoever receives the file link or attachment. Contracts, payroll records, tax files, and internal client documents are the common examples.
If your main problem is file size, use Compress PDF. If the goal is preventing edits rather than encrypting access, use other PDF controls. This page is specifically about encrypting the file with a password before sharing.
Password-protected PDFs add a practical extra layer when you need to send sensitive files over ordinary channels like email or chat while keeping access gated to the intended recipient.
In practice, protection usually happens at the end of a document flow: sign first with PDF e-signatures, label the file with confidential or draft watermarks, then encrypt the version that actually gets sent.
Open password vs permissions password
The PDF specification supports two distinct kinds of password, and they answer different questions. Confusing them leads people to protect the wrong thing.
Open password (user / document-open password)
Controls whether the file can be opened at all. Without it, the PDF is encrypted and completely unreadable — the reader shows a password prompt and nothing else until the right password is entered. This is the password that matters when you are sending a sensitive file to a specific recipient: it is what keeps the content unreadable if the email or link reaches the wrong person. For contracts, payroll, and medical records sent over ordinary channels, the open password is the protection you want.
Permissions password (owner password)
Leaves the file openable but restricts what a reader can do once it is open — for example printing, copying text out, or editing. The document is readable; the permissions password governs actions on it. It is useful when you want people to view a document but not, say, extract its text or print it. On its own it is a weaker control than an open password, because the content is still visible to anyone who receives the file.
Which one you actually need
If the goal is that the wrong person cannot read the document, you need the open password — encrypt the file so it cannot be opened without it. A permissions password addresses a narrower problem: a reader you trust to view the file but not to reprint or copy it. For most sensitive sharing — sending a tax return to an accountant, a contract to a counterparty, an HR file to a colleague — the open password is the one that does the job, because unreadable-without-the-password is the protection that counts.
Protecting a PDF vs unlocking one
Protecting and unlocking are the two opposite ends of the same operation — one adds the lock, the other removes it. They are anti-symmetric peers, and which one you need depends on the direction you are going.
Protecting a PDF, the subject of this page, adds a password and encrypts the file so it cannot be opened without that password. You do this before sharing something sensitive — the file leaves your phone already encrypted, and only someone with the password can read it.
Unlocking a PDF is the reverse: removing the password from a file you can already open, producing a clean unprotected copy. This is what you want when the gate is no longer needed — a bank statement you have already decrypted and want to archive without retyping a password every time, or a protected document a sender gave you the password for that you would rather keep open in your own library. You can only unlock a PDF if you know its password; there is no recovery for a forgotten one, by design. For that workflow, see unlock PDF on iPhone — the anti-symmetric counterpart to this page, which adds the lock that unlock removes.
