Is PDF redaction reversible? How to redact so text can't be recovered
Open a lot of "redacted" PDFs in a text editor and the hidden words are still right there, sitting under the black boxes. Whether a redaction can be reversed comes down to one thing: did the tool delete the text, or did it just paint a rectangle over it? If it painted a rectangle, getting the original back takes about ten seconds.
The usual ways people get the text back
Copy and paste is the classic. Select the page, paste it into any text editor, and the words that looked blacked out come straight through, because the black box is a separate object drawn on top of text that was never removed.
Often you don't even need that. In many PDF editors you can click the black rectangle, hit delete, and read what was underneath.
Then there's the stuff that isn't on the visible page at all: document metadata, comments, tracked changes, form fields, embedded thumbnails. A black box covers none of it. That is exactly how several well-known leaks happened, including court and government documents that were published with the redacted names still recoverable.
Test your own file before you send it
Open the finished PDF, select all the text on the page, copy it, and paste into Notes or any plain editor. If anything you meant to hide shows up, the file is not safe to share yet. Check the document properties while you're at it, since names and dates like to hide there too.
What it means to actually remove the text
Proper redaction deletes the content. The text, images, and vector shapes inside the box are removed from the file, the page is flattened so there's no separate layer to peel off, and the metadata is stripped. Once that's done there is nothing left to recover, because the data no longer exists in the document. This is the standard courts and hospitals are meant to use, for the obvious reason that a leaked account number or home address can cause serious harm.
Doing it so it can't be undone
Use a tool that removes content instead of drawing over it, then run the copy test above to be sure. Ours works in your browser, so the file is never uploaded and the redaction can't be pulled back out.
Redact a PDF for free, then try to copy the text back. You won't get anything.