This is the wrong thing to do and it's really important that you don't do it. Not only because it's wrong, but because you can end up with a PDF that is completely broken and useless.
A digital signature always applies to the whole document. There is no choice in this; it is your final seal on the document. It marks the page, but I think this was a mistake, because people sometimes trust the mark, which can be faked.
I suspect you need to see some marks because of company rules. To do this you first STAMP every page with some image (even a scanned signature). Finally you add one digital signature on the whole file.