Bates Numbering - Read Only File Error

I open a PDF attachment from an email in Adobe Acrobat XI. I the go to Tools, Pages, Bates Numbering, Add Bates Numbering. The Bates Numbering dialog box comes up. I choose Add Files, Add Open Files, select the file and click Add Files. I get a warning/error that read only files cannot have a bates stamp and in red type it tells me to remove the problem file from the list. I can save the attachment outside of Outlook and it works just fine going through the same steps. Has anyone seen this before? I would just use the workaround, but the user won't accept that as a solution.