That is what redaction is for. What makes you feel it could be undoable?
In Acrobat Reduction is a two-step process.
1. Tools->Protection->Mark for Reduction. In this step you mark all content that you want to reduct out.
2. Tools->Protection->Apply Reduction. Then save the reducted PDF (with a different name/location if you want to have the original available).
The Reduction tool in Acrobat permanently removes the reducted content from the saved PDF, so that it is not there. Because of that anyone who opens saved reducted PDF cannot undo reduction because the reducted content is not even in the file.
Make sure that you performed step 2. If you just performed step1 and saved the file, the to be reducted content is still there. This is useful wgen several people reduct the same PDF and want to see what has been already reducted.