How to Protect Sensitive PDF Documents Before Sharing Them

A PDF can leak in four different ways, and each one has a different fix. A password stops a stranger from opening the file. It does nothing about a bank account number that should never have been in the file, about a recipient who forwards it, or about a form field somebody edits after you signed. This guide walks through the four fixes in the order that works, and says plainly what each one does not do.

Match the risk to the fix

What could go wrong	What to do	Tool
The file holds data the recipient does not need (ID numbers, salaries, other clients)	Remove it for good	Redact PDF
Fields, stamps or notes could be changed after you send	Make them part of the page	Flatten PDF
The file gets forwarded and nobody knows whose copy it was	Mark every page	Watermark PDF
Someone other than the recipient opens it	Encrypt it with a password	Protect PDF

1. Remove what the recipient does not need

Start here, because nothing protects data better than not sending it.

Drawing a black rectangle over a line in a PDF editor is not redaction. The text is still in the file underneath: anyone can select the area, copy it and paste it into a note, or simply delete the rectangle. Real redaction deletes the content itself.

In Redact PDF you upload the file, mark the text or the areas to remove, and download a copy in which that information is gone. Only what you marked is removed; the rest of the document keeps its content and layout. Typical candidates are national ID and passport numbers, bank account numbers, home addresses, signatures of third parties and prices meant for another customer.

Check the result before you send it: open the redacted copy, try to select and copy the blacked-out area, and search the file for one of the removed numbers. Both should find nothing.

2. Flatten filled forms, stamps and notes

A filled form is still a form. The recipient can click into a field and type a different amount, or delete a comment. Flattening merges form fields, annotations and layers into the page, so what you send is what stays. Do this after filling and signing, and keep the original if you may need to edit later: a flattened file cannot be turned back into a form.

3. Mark the copy

A watermark does not stop anyone from reading or forwarding a file. It makes a forwarded copy recognisable, which is often enough to stop casual sharing.

Watermark PDF stamps a line of text across every page, at the size, angle, opacity and colour you choose. CONFIDENTIAL and DRAFT are the usual choices. For a document that goes to several people, the recipient's name or company works better: each person receives a copy that points back to them. Keep the opacity low enough that the text behind stays readable. The tool stamps text only; to place a logo or a stamp image on a page, use Sign PDF.

4. Encrypt the file with a password

Do this last. An encrypted file has to be unlocked before it can be redacted, flattened or watermarked.

In Protect PDF you upload the file, set the password needed to open it and download the encrypted copy. AES-256 is the level to pick unless the recipient uses very old software. You can also set a separate owner password and switch printing or copying off. Treat those switches as a request, not a lock: they rely on the reader application honouring them. What actually keeps the content from a stranger is the password needed to open the file.

Three rules make the password worth having:

1. Make it long. Four words you can say over the phone beat eight random characters that end up in the same email as the file.
2. Send it another way. If the file goes by email, send the password by phone, SMS or a messaging app. A password in the same message protects against nothing.
3. Store it before you send. Confileo does not keep your password, so it cannot be recovered. If you lose it, the protected copy stays closed; keep the unprotected original somewhere safe.

Before you press send

• Open the final file on another device and confirm it asks for the password.
• Check that the redacted areas cannot be selected or found by search.
• Send the password through a different channel than the file.
• Keep the original, unprotected and unredacted, in your own storage only.

Questions people ask

Does a password stop the recipient from sharing the file?

No. Anyone who has the password can open the file, save an unprotected copy and pass it on. A password decides who can open the file, not what they do next. A watermark with the recipient's name is the practical deterrent.

Is hiding text with a black box safe?

No. A shape drawn over text leaves the text in the file. Use redaction, which deletes the underlying content, and test the result by trying to copy from the blacked-out area.

Can I remove the password later?

Yes. Anyone who knows the password can open the file and save it again without protection. Without the password there is no way back, which is the point of encrypting it.

Which should I do first, the watermark or the password?

The watermark. Finish every change to the content first — redaction, flattening, watermark — and encrypt the finished file at the end.

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Originally published on the Confileo blog.