How to Password-Protect a PDF Online — Free, AES-128, No Watermark

You drafted a contract, a salary letter, a payroll spreadsheet. Now you need to email it to one specific person without worrying about it being forwarded to twelve others. Add a password to the PDF before you send it — the recipient unlocks it, the wider world can't. Here's how, in fifteen seconds.

When you actually need a PDF password

Real cases: emailing payroll spreadsheets to accounting, sending a salary offer letter to a candidate, attaching a tax return to a loan application, shipping board materials to directors, forwarding medical records to a specialist, or any document that'd be a problem if the wrong person opened it.

A password on a PDF doesn't prevent forwarding — anyone with the password can still re-share the file. What it does prevent is accidental exposure: an email going to the wrong recipient, a forgotten USB stick, a backup ending up on a cloud bucket. The recipient has to know the password to read the file, and you control who has the password.

The rule we tell every customer

Send the file and the password through two different channels. Email the PDF, text the password. Slack the PDF, WhatsApp the password. Same channel = no protection if the channel is breached.

How to password-protect a PDF, step by step

Open the Protect PDF tool

Go to convertpdfgo.com/protect. No account, no email, no installation. Loads in under a second.

Drop the PDF in

Drag the file onto the upload area or click to pick it. Files up to 30 MB go through without a queue. Encrypted in transit (TLS 1.3), encrypted at rest, deleted within one hour.

Type a password

Pick something you'll remember and the recipient can be told. Aim for 12+ characters mixing case + numbers (e.g. Mango-7-Truck). Avoid words from the document itself — "Q4Sales2026!" is what someone trying to guess would try first.

Click Protect

We encrypt the PDF with AES-128 — the same algorithm Adobe Acrobat uses by default. Output is a normal-looking .pdf that prompts for the password whenever it's opened.

Send password separately

Email the locked PDF. Text the password. The recipient enters the password in their PDF reader and the file opens.

What AES-128 actually protects against

Strong: random brute-force attempts. AES-128 has 2^128 possible keys — even with billion-attempts-per-second computing, guessing one would take longer than the age of the universe. Practically unbreakable from a math standpoint.

Strong: accidental exposure. If the file lands in someone's inbox by mistake, they see "PDF requires password" and can't poke around.

Weak: weak passwords. Encryption only protects what the key protects. If the password is password1 or the company name + year, any half-decent attacker breaks it in seconds with a dictionary attack. The encryption math isn't broken — the password is.

Weak: forwarding by an authorised recipient. Once the legit recipient opens the file, they can save it, screenshot it, forward it. AES protects against eavesdroppers, not against people who know the key.

Common protection mistakes and how to avoid them

Sending the password in the same email

If the email is intercepted, both the file and the key go together. Defeats the purpose entirely. Use a separate channel — SMS, Signal, in-person.

Using the company name + year as password

"Acme2026", "Acme!", "Acme@2026" are the first 10 things an attacker tries. Use a passphrase with random words instead: "Mango-7-Truck" is stronger than "Acme2026!" and easier to remember.

Forgetting the password yourself

AES is unbreakable, which means you also can't recover it. Save the password in a password manager (1Password, Bitwarden, Apple Keychain) the moment you set it. If you lose it, the file is permanently locked.

Re-protecting an already-protected PDF

You can't add a second password to a file that's already locked. Remove the first password via our unlock tool (using the original password), then re-protect with the new one.

A short word on privacy

Three things to know. One: the file and the password go over TLS 1.3. Two: files are encrypted at rest while we process them, then deleted within one hour. We never log the passwords you type. Three: once the locked PDF is downloaded, the password lives only with you and the recipient — we don't store it anywhere.

One last reminder

We can't recover lost passwords. Save them in a password manager the moment you set them. Lost = locked forever.

How we compare to other free PDF protection tools

Feature	convertpdfgo	Smallpdf (free)	iLovePDF (free)
Encryption	AES-128	AES-128	AES-128
Sign-up	None	After 2/day	Optional
Watermark	None	None	None
Password logged?	Never	Vague	Vague
Auto-delete	1 hour	Vague	2 hours

Frequently asked questions

Will Adobe Acrobat / Preview / Chrome respect the password?

Yes — any standards-compliant PDF reader prompts for the password before showing the content. Adobe Reader, Preview, Chrome, Edge, Firefox, mobile readers all honour AES-128 PDF passwords.

Can I lock just printing but allow reading?

Not in this tool — we set a single "open password". For permission-only locks (print / copy / edit), Adobe Acrobat Pro is the standard.

What's the maximum password length?

128 characters. We recommend 12+ characters with mixed case + digits.

Does it work for scanned PDFs?

Yes — encryption protects the file bytes, not the content type. Scanned PDFs lock the same as text PDFs.

How long does it take?

About 1 second per MB. A 5 MB PDF takes ~5 seconds.

Is there a usage limit?

No. Free, no daily cap, no sign-up wall, no watermark.

Can I unlock the PDF later?

Yes — use our Unlock PDF tool with the same password. The tool removes the encryption, the text returns to plain readability.

What to do next

One-click follow-ups

After protecting, you can sign the PDF first, stamp it CONFIDENTIAL, compress before emailing, or unlock it later when you don't need the password. All free.

Or browse our full list of 49 free PDF tools.