The black rectangle that is only a drawing
In 2011 a court filing about a well known technology dispute was published with sections blacked out, and within hours people had the text: the blocks were rectangles drawn over a page whose words were still underneath, and selecting the region and pasting it revealed everything. The same mistake has been made since by government departments, law firms and newspapers, and it will be made again next month. It is not a rare accident. It is the default outcome of the most obvious way to do the job.
The reason is structural. A PDF is a collection of objects: text-showing operators with their fonts and positions, images, vector paths. Drawing a filled rectangle adds one more object to that collection, on top in painting order. It changes what a viewer displays and nothing about what the file contains. The text operators are untouched, so text extraction returns them, copy and paste returns them, a screen reader reads them aloud, and any of a hundred libraries can list them in a line of code. The document looks redacted to a human and is not redacted at all to a program.
Genuine redaction has to remove the objects, and there are two ways. The precise one is surgical: parse the content stream, find the text-showing operators whose glyph positions fall inside the region, and rewrite the stream without them. It preserves the rest of the page as real text and it is genuinely difficult, because glyphs are positioned by a running matrix that later operators depend on, fonts may be subset so removing the last use of a glyph can invalidate the font, and a partially covered word has to be split rather than dropped.
The blunt one is to rasterise. Render the page to pixels at a sensible resolution, paint the boxes onto those pixels, and build a new document with the result as an image. Every text object in the original is gone, not because it was carefully removed but because nothing from the original structure survived the trip through a bitmap. There is no content stream left to mine. This is what the tool does, and the honesty of it is the point: it is impossible to get subtly wrong, which is exactly the failure mode the surgical approach has.
The trade is real and worth stating plainly. The output cannot be searched, selected or read aloud, and it is larger. A redaction you can still search is a redaction that still contains the text, so those two properties cannot both be had for the same words. What you can have is a document where the words that mattered are unrecoverable, which is the requirement people actually have when they reach for this. Rounding also matters more than it sounds: the boxes are rounded outward to whole pixels, because a box that rounds inward can leave a one pixel line of the original along its edge, and a redaction that leaks a sliver of what it hid is worse than none, since it looks finished.