Truly delete the areas you specify (real content removal, not just a visual box — see FAQ).
This is true redaction — the marked page is converted to an image and the underlying text/image content underneath is completely erased, not just covered with a box.
No, since redacted pages are converted to images, their text can no longer be searched or copied — this is an accepted trade-off for guaranteeing true concealment.
You need to enter x/y/width/height values in the PDF page's own units (points, with the origin at the bottom-left corner).