A contract just landed in your inbox and you need to forward it to a colleague — but it has bank details halfway down page three. Emailing it raw feels wrong. A password on the file is the bare minimum a confidential PDF deserves.
The good news: you don't need Acrobat to do it, and you don't need to install anything.
What a PDF password actually does
When you add a password to a PDF, the contents are encrypted — the file becomes unreadable bytes until the correct password is entered. Anyone who intercepts the file on email, finds it on a shared drive, or pulls it off a lost USB stick sees gibberish. That's the whole point.
There are two kinds of password: a user password (also called an open password) controls who can view the file, and an owner password controls who can change permissions like printing and editing. For most people, the user password is what matters.
Locking a PDF in Flint
Open Flint's password tool, drop the PDF in, type a password, and download the encrypted copy. The original file never lives on a server you can't see — Flint runs in your browser. That matters when the document is anything you wouldn't tape to a noticeboard.
If you'd rather lock a PDF *after* editing it, you can edit the PDF first and apply a password as the final step. Same with merging two contracts — finish the document, then encrypt the result.
How to share the password safely
The cardinal rule: never send the password in the same email as the file. If an attacker gets the email, they get both. Send the file by email and the password by text, Signal, or a quick phone call.
For anything sensitive — payroll, medical, legal — assume the email itself may be archived, scanned, or forwarded. Separating channels means a single breach doesn't hand someone everything.
Choosing a password that actually holds
PDF passwords are cracked by brute force. A six-character password falls in minutes. A passphrase of four random words takes centuries. Use a password manager and generate something long — 16+ characters, mixed case, numbers and symbols, or a memorable passphrase like `correct-horse-battery-staple-93`.
If you'll need to unlock the file again yourself, store the password in 1Password, Bitwarden or your team's vault. Don't email yourself the password — that defeats the entire exercise.
FAQ
Does Flint store my PDF when I add a password?
No. Encryption happens in your browser. Your file isn't uploaded to a server that keeps it.
Can I add two different passwords to one PDF?
Yes — a user password to open the file, and an owner password to control printing, copying and editing. Both can be set in one go.
Is a password the same as a digital signature?
No. A password controls access. A digital signature proves who signed it and that nothing has changed since. They solve different problems and are often used together.
What if I forget the password?
There's no back door. If you lose the password to a PDF you encrypted, the file is effectively gone. Always store passwords in a password manager.
Confidential documents shouldn't travel naked. Lock yours with Flint's password tool before it leaves your machine.