Redacting PDFs Properly — A Black Box Is Not Redaction

1. The most common mistake

When hiding an ID number on a contract or a scanned document, many people draw a black rectangle over it in a PDF editor. This is not safe.

A PDF is not a picture; it is a collection of objects. The black rectangle is a separate shape object layered on top, and the text object underneath remains in the file. As a result:

Selecting and copying that region pastes the original text.

Any text-extraction tool returns the hidden content.

Deleting the rectangle object reveals the original.

Government bodies and large law firms have repeatedly leaked confidential information exactly this way.

2. What actually works

Rasterise the page, then cover. Converting the page to an image removes the text layer, so nothing survives beneath the mark. You lose searchability and gain file size, but the result is definitive.

Use a real redaction feature. Proper redaction removes the underlying objects rather than covering them.

Send only the pages you need. Excluding the sensitive page in the first place is the simplest and most reliable option.

In practice the third approach is usually best. If only 3 of 20 pages are needed, the redaction problem for the other 17 simply disappears.

3. Information you cannot see

Beyond the visible content, a PDF can carry:

Metadata: author name, organisation, producing application, creation and modification timestamps

Revision history: files edited via incremental updates can retain earlier versions

Attachments and annotations: present in the file even when not displayed

Cropped regions: cropping a page sometimes hides content rather than deleting it

Check document properties and strip metadata when it is sensitive.

4. Risks of upload-based online tools

Many free PDF services upload your file to their servers. Before using one, check:

When files are deleted, and whether a retention period is stated

Whether result URLs are guessable

Whether the privacy policy allows third-party sharing

For employment contracts, medical records, or bank documents, avoiding the upload altogether is the safer default.

5. How to verify local processing yourself

The claim "your file is never uploaded" is testable.

Open developer tools (F12) and switch to the Network tab.

Select a file and run the operation.

Look for any upload request carrying the file's payload.

With genuine local processing, no such request appears. The EVEDAYS PDF Tool works this way, so you can confirm it directly.

To be precise, local processing removes exposure in transit and server-side retention. It does not defend against malware already present on your own device.

6. Practical checklist

Keep only the pages the recipient actually needs.

For anything left to hide, rasterise first or use real redaction.

Reopen the result and try to select and copy the hidden region.

Check document properties and clear author and organisation metadata.

Make sure the filename itself does not contain a name or ID number.

Steps 4 and 5 are skipped surprisingly often.

7. Sources

ISO 32000-1 — Portable Document Format

Personal Information Protection Commission of Korea