Why not just draw a black rectangle in an editor?
Because a drawn rectangle only covers the text. Anyone can select underneath it or delete the shape. News stories about "redacted" documents leaking come from exactly that mistake. Rasterizing destroys the data itself.
Can the redacted content be recovered?
No. The marked regions exist only as black pixels in a re-rendered image; the underlying text and the original metadata are not present in the output file.
Is the document uploaded for processing?
No, which matters more here than anywhere: the whole point is sensitive content, and it never leaves your browser.