How it works
Why pages become images. A black box drawn over live text looks redacted and is not: Select All, search or any PDF editor hands the words straight back. So matching pages are painted, then rendered and rebuilt from the picture. The text objects no longer exist.
Phrases across runs. "Jane Doe" is usually three glyph runs in a PDF, sometimes more. The page is flattened into one searchable string first, so a phrase is found even across a line break.
Zero matches is an error. A redaction tool that silently returns your file unchanged is the most dangerous thing it could do, so it does not.