The Black Box Over Your PDF Is Not Hiding Anything
This is one of the most expensive misunderstandings in document handling, and it looks completely convincing on screen. You open a PDF, drag a black rectangle over a name or an account number, save it, and send it off believing the information is gone. It is not gone. In most cases it is sitting right there under the box, one copy-and-paste away from whoever you sent the file to.
It is not a rare edge case either. Courts, government agencies, and large companies have all published documents with black boxes that anyone could see straight through, because the box was a drawing on top of the page and the text layer underneath was never touched.
See it for yourself
Below is a line from a pretend document. Cover it with a black box, then try to copy what is hidden, and watch what an attacker would actually get. Then do the same with true redaction.
Why the box hides nothing
A PDF keeps its text as real characters in a layer, separate from anything drawn on the page. When you add a black rectangle, you are painting over the surface, but the text layer under it is unchanged. Search still finds those words. Select-and-copy still lifts them. Delete the rectangle and they reappear. The picture looks redacted; the data never was.
How to check a file you already have
You can test any PDF in about ten seconds. Open it, try to highlight the text inside a blacked-out area, copy it, and paste it into a notes app. If words show up, the redaction is fake. As a second check, use the find function and search for a name or number you expected to be removed. If the search jumps to it, it is still in there, box or no box.
How to redact so it actually sticks
Real redaction removes the underlying content instead of hiding it. The characters, the images, the vectors beneath each marked area are deleted, so there is nothing left to copy or search. That is what OptaPDF Redact PDF does: it erases the content under every box rather than drawing on top, and the file is processed over an encrypted connection and deleted from our server within 30 minutes. Whatever tool you use, run the copy test on the result before you trust it.
One more thing that catches people even after a clean redaction. Hidden metadata, such as the author name, the software that made the file, and edit timestamps, sits in the file properties, not on the page, so your redaction boxes never touch it. If the document is sensitive, check the properties and clear anything revealing before it leaves your hands.
The short version
A black box in a PDF is a costume, not a delete key. If it was drawn on top, the text is still in the file and anyone can copy it back out, which is exactly how so many "redacted" documents have leaked. Test your own files with the copy trick, redact with a tool that removes the content rather than covering it, and clear the hidden metadata while you are at it. Then the thing you meant to hide is actually gone.