Deleting a cover page to a PDF bars Adobe from reading the PDF when re-open.

I received PDFs that usually have a cover page that is not relevant to the rest of the document. (For instance, a cover page reading, "Here is the agreement.") I want to delete the coverage page and save the documents. I can, but after I do, in either Adobe or Apple Preview - it does not matter - Adobe cannot reopen the document. I get the following "errors":

The document's page tree contains an invalid node.

There was a problem reading this document (14).

Any solutions?

Thank you to any, and all that can help!