Payslips, contracts, tax returns and medical records travel by email and sit in shared folders every day. Anyone who gets hold of the file can read it, unless it has a password. Adding one takes less than a minute.
The catch with most online tools is that you have to upload the very file you want to keep private, together with its new password. This guide shows a way to do it where neither ever leaves your computer or phone.
Step by step
- Open the tool. Go to pdfmerger.io/protect-pdf. There is nothing to install and no account to create.
- Add your PDF. Drag it onto the page or click to choose it.
- Type a password twice. The second box catches typos, so you don't lock yourself out.
- Choose restrictions (optional). Untick printing, copying or editing if you want to block them for anyone who opens the file.
- Click "Protect PDF". The encrypted copy downloads straight away.
Private by design
Your PDF and your password are processed inside your browser. Nothing is uploaded, so there is no copy on a server that could leak.
How strong is the protection?
The file is encrypted with AES-256, the strongest method in the PDF standard (PDF 2.0, security revision 6) and the same level Adobe Acrobat uses. Without the password the content is unreadable: it is not hidden, it is scrambled.
That strength depends on your password. A long passphrase such as four random words is far harder to guess than a short word with a number on the end.
Open password vs. restrictions
- Open password: nobody can see a single page without it. This is what protects the content.
- Restrictions: block printing, copying text or editing for people who can open the file. Add an optional owner password if you want to lift them later.
Restrictions are respected by mainstream readers such as Acrobat, Preview and Chrome, but they are a polite request rather than a lock. For real secrecy, rely on the open password.
Sending the password safely
Don't put the password in the same email as the file. Send it by text message, say it on the phone, or agree on it in advance. Otherwise anyone who sees the email has both halves.
If you forget the password
It can't be recovered, by us or by anyone else, because it is never stored anywhere. Keep it in a password manager, and keep an unprotected original somewhere safe if you might need it.
When you no longer need the protection, open the file in Unlock PDF, type the password once and download an unprotected copy.
Frequently asked questions
Does the protected PDF open on every device?
Yes. Adobe Acrobat, Apple Preview, Chrome, Edge, Firefox and the built-in readers on iPhone and Android all ask for the password and then open the file normally.
Can I protect a PDF on my phone?
Yes. The tool works in Safari and Chrome on iPhone and Android, with the same encryption.
Is it really free?
Yes. No sign up, no watermark and no daily limit.
Lock your file before you send it: pdfmerger.io/protect-pdf.