"there" not "their" - unless I am really confused about what you are doing !
I put a white rectangle over what I wanted to crop out of the pdf I was working on.
Not a proper solution, i then needed to use "export as PDF" of my PDF so that the next user of the document couldn't simply lift off the rectangle I had put on the document to cover what ideally I would have removed.
Add mark up is something else I tried, to black out the text that I wanted concealed, but we are limited to a slim choice of colours, not including black.
I would like to see a function "crop" with "crop inside" and "crop outside" options. I am not familiar with Adobe or other software. Presumably since they invented it, Adobe is the place to go if you have repeated exercises requiring meddling with Adobe documents.