Why is text still readable under a black box in a PDF?

Drawing a rectangle over text adds a shape to the page. It does not remove the text, which stays in the content stream and can be selected, copied or extracted by any PDF library. Genuine redaction deletes the text first and then draws the box; the two look identical on screen.

How does this tell a redaction box apart from a highlight or table shading?

By paint order. A table's shading and a highlight band are painted before the text that sits on them, which is the only way that text is visible. A shape painted after the text hides it. Translucent fills are ignored, and so are page-sized shapes, which are watermarks rather than redaction boxes.

Is my document uploaded or stored?

The file is sent over HTTPS, parsed in memory and never written to disk. Any recovered text is returned in the response and kept nowhere.

How do I redact a PDF properly?

Use a tool's redaction feature and then apply the redactions, which removes the underlying content. Exporting to flattened images also works, at the cost of searchable text. Drawing a black rectangle, using a highlighter in black, or placing a filled shape over text does not remove anything.

Is the text under your black box still there?

Drop a PDF here, or browse

Max 20 MB · processed in memory, never saved

No PDF handy? Try the sample:

Two fictional documents. The memo has a name and a settlement figure under black bars; the statement has neither, and is here so you can see what a silent report looks like.

What this finds

Two failures — different mistakes, same outcome.

Text under a covering shape

Someone drew a rectangle over a name. The rectangle is a new object on the page; the name is exactly where it always was. Select the area in any reader, run pdftotext, or open the file with any PDF library and the words come straight out. The report hands them back, because if this checker can read them so can anyone who receives the document.

Redaction marks that were never applied

PDF has a real redaction annotation. It is a mark — a request that a region be redacted — and the content only leaves the file when a tool applies the marks. A document sent between those two steps looks finished, because most viewers draw the marks as solid boxes, and is not.

How it decides

By paint order, which is the one thing separating a redaction box from ordinary design. A table's shading, a highlight band and a coloured header are all filled rectangles sharing coordinates with text — and every one of them is painted before that text, because that is the only way the text ends up visible. A rectangle painted after the text is on top of it, and hiding it.

The same two objects, in the two possible orders

draw shading          background — the text lands on top, and reads normally
draw text

draw text             cover — the text is underneath, and still in the file
draw black box

Three further rules keep it quiet on honest documents. Translucent fills are ignored, because a highlighter hides nothing. Shapes larger than 40% of the page are treated as watermarks and stamps rather than redactions. And invisible text — the OCR layer under a scan — is excluded, since it is designed to be unreadable on screen and extractable by machine.

What it cannot see

A checker that implies more coverage than it has is worse than none. Four things this one misses, stated plainly.

Boxes inside form XObjects. Reusable content blocks are not walked, so a covering shape inside one is missed. When a page uses them the finding says so, and the count is a floor rather than a total.
Flattened documents. If the page was exported to an image there is no text layer left to find — which also means the redaction worked.
Fonts with no character map. The coverage is still reported, but the words may not be recoverable by this tool. Someone with the font programs can still get them.
Everything outside the page content. A name taken off the visible page can survive in document metadata, an attachment, a form field or an earlier revision. Those are separate signals in the full report.

How to redact so that it holds

Use your tool's redaction feature and then apply the redactions — the step that deletes the underlying content. Exporting to flattened images also works, and costs you searchable text. What does not work: drawing a filled rectangle, using a black highlighter, setting the text colour to match the background, or covering the area with an image. All four leave every character in the file.