Why this gesture almost always fails
A PDF is not an image: it is a list of drawing instructions in which text keeps its characters and its coordinates. A box added on top is one more instruction, executed after the others. On screen it covers; in the file it erases nothing. A copy and paste, a text extraction or a plain search brings the covered word back in a second. That is how patient names, salaries and addresses resurface from reports published as redacted.