Lock PDF — Encrypt a PDF into a Password-Locked File

This encrypts your PDF into a locked file — it does not password-protect the PDF

That distinction is the whole tool, and it is worth spending a moment on because it changes what you should expect. When people say “password protect a PDF,” they usually mean the thing Acrobat and most desktop tools do: the file stays a PDF, but opening it pops up a password box, and once you type the password it opens in whatever reader you already have. The file is still recognisably a PDF the entire time.

This tool does something different. It takes your PDF, encrypts the entire file as one opaque blob, and wraps it in a small ToolMintX-specific container saved with a .tmx extension. There is no password box, because there is no PDF for a reader to look at — from the outside it is just encrypted bytes. Double-click it and Adobe Reader, Preview, Chrome, and every other viewer will say it is damaged or unreadable, because as far as they are concerned it is not a PDF at all. The only thing that can turn it back into your document is ToolMintX Unlock PDF with the same password.

So the honest one-line description is: this is a private encryption box for a PDF you will unlock later with the same tool, not a way to hand someone a PDF they can open with a password in their own reader. If your goal is the second thing — a recipient who does not use ToolMintX types a password and reads the document — the built-in password feature in Acrobat, Word’s export-to-PDF, or your operating system’s print-to-PDF is what you want, because those produce a standards compliant encrypted PDF. This tool is for the case where you control both ends: you lock a file to store or move it safely, and you unlock it here when you need it back.

What the encryption actually is

The cryptography under the hood is deliberately boring, which for encryption is a compliment. When you click Encrypt PDF, the tool takes the password you typed and runs it through PBKDF2— Password-Based Key Derivation Function 2 — using SHA-256, a fresh random 16-byte salt, and 100,000 iterations. PBKDF2’s job is to turn a human password, which is usually short and guessable, into a 256-bit key while making each guess expensive. The 100,000 iterations mean an attacker trying to brute-force the password has to do that whole stretch for every attempt, which is what turns “a few million guesses a second” into something far slower. The random salt means two people who choose the same password still get completely different keys, so precomputed lookup tables are useless.

The derived key then encrypts the file with AES-256 in GCM modeunder a random 12-byte initialization vector. AES-256 is the same symmetric cipher used across banking, messaging, and government systems; GCM is the mode that also produces an authentication tag, so the decryption step can tell the difference between “correct password” and “wrong password or tampered file” instead of silently returning scrambled bytes. That is why entering the wrong password in Unlock PDF gives you a clean “incorrect password” message rather than a corrupt download.

All of this runs through the browser’s built-in Web Crypto API. That matters for two reasons. First, it is a vetted, native implementation rather than hand-rolled JavaScript crypto, so the actual encryption is done by the same code the browser uses everywhere else. Second, browsers only expose Web Crypto in a secure context — over HTTPS or on localhost. If you somehow open this page over plain HTTP, the tool does not fall back to a weaker cipher to keep working; it stops and tells you to switch to https. That refusal is intentional: a silent downgrade would leave you believing you had AES-256 when you did not.

Inside the locked file

The .tmx file is not mysterious once you see its layout. It is four regions laid end to end. The first four bytes are a marker — the ASCII letters TMXL — which is how Unlock PDF instantly knows this is a ToolMintX AES envelope rather than some other file. The next sixteen bytes are the random salt used for PBKDF2, and the twelve bytes after that are the AES-GCM initialization vector. Everything from byte 32 onward is the ciphertext: your encrypted PDF plus the GCM authentication tag.

Storing the salt and IV in the clear alongside the ciphertext is normal and safe — neither is secret. Their only job is to make sure the same password produces a repeatable key (the salt) and that the encryption is unique per file (the IV). Without your password they reveal nothing; with your password they are exactly what Unlock PDF needs to rebuild the key and reverse the process. There is no copy of the password anywhere in the file, which is precisely why a forgotten password is unrecoverable.

One consequence of this design worth naming: the format is proprietary. A standards-compliant encrypted PDF can be opened by any conforming reader that knows the password, but a .tmx envelope can only be opened by a tool that understands the TMXL layout — in practice, ToolMintX Unlock PDF. That is a real trade-off, not a hidden one. You gain a simple, whole-file encryption that does not depend on the quirks of PDF’s own (historically weak) encryption filters, and you give up the ability to hand the file to someone who just wants to open it in Acrobat. Which side of that trade is right depends entirely on your use case, and the next section is about choosing.

When this is the right tool, and when it is not

Reach for this when both ends are you, or people who will use ToolMintX. Encrypting a sensitive PDF before it sits in a cloud folder, on a shared drive, in an email you are wary of, or on a USB stick you might lose is a good fit: the file is unreadable to anyone who finds it, and you can turn it back into a PDF here whenever you need it. Archiving your own confidential records — contracts, statements, identity documents — in encrypted form is another natural fit, provided you keep the password safe and, ideally, keep an unencrypted master copy somewhere else secure so a lost password is an inconvenience rather than a disaster.

Do not reach for it when the recipient just needs to open the document. If you are sending a client an invoice they should be able to view by typing a password in their own reader, a .tmx file will only frustrate them — they will double-click it, their PDF viewer will reject it, and you will end up walking them through installing nothing helps because it is not a PDF. For that, use the password option built into whatever created the PDF, which produces a normal encrypted PDF. Likewise, if you need the recipient to fill a form, sign, or print through a portal that expects a real PDF, an encrypted envelope is the wrong shape entirely.

And a plain reminder that applies whichever tool you choose: encryption protects the file, not your habits. A strong password on a document you then email alongside the password in the same thread protects nothing. Send the file and the password by different channels, and treat the password like the key it is.

Choosing a password that is worth the encryption

AES-256 is not the weak link — the password is. Because the whole scheme derives its key from what you type, the encryption is only as strong as the password is hard to guess. The strength meter beside the field is a rough guide: it rewards length, a mix of upper and lower case, digits, and symbols. Treat “Strong” as a floor, not a finish line. A short password with a symbol bolted on is still short; a long passphrase of a few unrelated words is both stronger and easier to remember than a scramble of eight characters.

The single most important habit is to record the password before you delete the original. There is no reset link, no security question, and no support ticket that can recover the file — that is the direct cost of the file and password never leaving your browser. A password manager is the right home for it. If the encrypted copy is going to be your only copy, double-check you can actually unlock it — run the .tmx file through Unlock PDF once, right after you make it, before you throw the original away.

Related tools

The natural companion is Unlock PDF, which reverses this process with the same password and also handles some standard password-protected PDFs. If your real goal is to strip hidden information rather than lock the file — author names, edit history, and other properties — the PDF metadata remover is the tool for that, and it leaves you with an ordinary, openable PDF. And if the file is large before you encrypt it, running it through the PDF compressor first keeps the encrypted envelope lean, since encryption preserves the input size and does not compress.