Why black boxes fail
Redaction leaks make the news regularly — court filings, government reports and corporate disclosures where the covered text was recovered in seconds. The cause is nearly always the same, and it is a misunderstanding of how a PDF is put together.
A PDF holds a list of drawing instructions. Adding a black rectangle appends one more instruction: draw a black rectangle here. It does not touch the instruction that says draw the word "Sharma" at this coordinate. That instruction is still in the file, still in the reading order, and selecting over the black box and copying will paste the text right back out.
The test anyone can run: open the redacted PDF, select all, copy, and paste into a text editor. If the supposedly hidden words appear, the redaction failed. Do this before sending any document you believe you have redacted — whatever tool made it.
How to redact properly here
- Cover everything sensitive with the Erase tool. Colour does not matter; use black if you want the redaction to be visible as such.
- Also clear any text you edited rather than covered, since the original wording sits under those patches too.
- Press Flatten instead of Save. Choose a resolution — 150 dpi for screen, 300 dpi for print.
- Each page is re-rendered as an image, so the saved file contains only visible pixels. Nothing lies underneath.
What flattening costs you
| Normal save | Flatten | |
|---|---|---|
| Text selectable and searchable | Yes | No |
| Still editable later | Yes | No |
| Hidden text removed | No | Yes |
| File size | Close to the original | Larger — pages become images |
| Screen reader accessible | Yes | No |
Because flattening removes the text layer, it also removes accessibility. Use it for the version you send out, and keep an unflattened copy for your own records.
What this does not cover
Flattening removes page content, and metadata is a separate matter. Author name, creation software and similar fields live outside the page and are not rendered, so check them separately if a document is going somewhere sensitive. Attachments, embedded files and revision history in the source document are likewise beyond the scope of a page-level tool.
Questions
Is a black rectangle ever enough?
Only if the text underneath was already removed. On its own, a rectangle is a visual cover and the words remain in the file for anyone who looks.
Does flattening remove metadata too?
It removes the page text, because the pages become images. Document metadata such as author and producer is stored separately and should be checked on its own.
What resolution should I choose?
150 dpi is comfortable for reading on screen and keeps files reasonable. Choose 300 dpi if the document will be printed or if it contains fine print and small figures.
Is my file uploaded during redaction?
No. Everything, including flattening, happens in your browser. A redaction tool that uploads your document to a server is a strange thing to trust, and this one never does.
Start editing
The editor opens instantly and runs on your own machine. Nothing is uploaded, and there is no account to create.