What OS you both work on?
Does this happen with any other document as well or just this one?
Please check if Acrobat' are updated to the latest patch.
They are both working on Windows 7 SP1 the issue happens intermittently with various documents not just one particular document. I will check if Adobe DC 2015 is updated to the latest patch.
What can you see when you add pages, save, close, and re.open the document?
The document is legible it is just missing the pages that were added. Since the issue is intermittent I am wondering if the issue is human error.