how to delete the same thing from all pages

I have opened a 500+ page pdf and need to edit out something from the header and footer from each and every page.
It is the same exact thing.
Normally, on a doc file, I would just find&replace finding the thing and then replacing with spaces.
However, select all doesn’t seem to select everything from all pages only the one that I am on.
Next, I don’t see any way to “combine” all the pages so that I can erase/modify the same thing.
Is there a way to delete the same phrase from all the pages or at least have a “find” function so that I can keep pressing next and next having it preselect the phrase so I can delete them from all the pages?
Thanks so much for the help.

Have you looked at question 249419?