Cover text in a PDF before you send it
A black box you can delete is not redaction. Mark the private area, download, and check the words cannot be selected.
Drawing a black rectangle in an ordinary editor looks covered and is often not. Someone can delete the rectangle and the sentence is still in the file. Redact PDF rebuilds the pages you mark as images, with the boxes painted solid black, so those pages no longer contain the original text layer. Pages you do not mark are copied as they were.
How to check the result
Open the download and try to select the words under the box. If you can still highlight an account number, the cover failed and you should not send that file. Also look at the page before and the page after. A marked page becomes a picture, which is why the words are gone. An unmarked page stays a normal PDF page, so a secret you forgot to box is still there.
Screenshots are a different job
A chat screenshot is not a PDF. Cover part of an image blurs or paints a rectangle and exports a new picture. It does not change the original file still sitting in your gallery, and it does not strip hidden metadata from that original. Delete the original yourself if you do not want to keep it.
Neither tool is a certified redaction review. Use them to remove a number, an address, or a name you can see. Do not use a signature stamp as a substitute: placing a signature image only paints a picture of a signature. It does not lock the file. The work stays in your browser, and the original on your disk is not overwritten.