I’ve unredacted proposals using the ‘unflatten’ command in Bluebeam Revu (which is by far the best PDF editor) which allowed me to underbid my competitor and win the job (and at a higher price than I would’ve submitted).
Definitely an ethical grey area, but an edge is an edge ;)
I'd not do this if I were you.
There are zero legal implications, it was a private contract. My customers regularly tell me the exact price that my competitors have submitted to them and that isn’t illegal.
Probably there are legal implications for attorneys circumventing redaction in legal documents but construction proposal letters have no protections against unredaction.
Legally, I can't see what's wrong with using information that you have, even if the other party didn't intend for you to have it. Lawyers themselves will use information in court that was accidentally sent to them by a counter-party, and that the other lawyer never intended them to have.
You've certainly piqued my curiosity. Can you say any more?