The issue for me was that DocumentFormat.OpenXml.dll
existed in the Global Assembly Cache (GAC) on my Win7 development box. So when publishing my project in VS2013, it found the file in the GAC and therefore omitted it from being copied to the publish folder.
Solution: remove the DLL from the GAC.
%windir%\Microsoft.NET\assembly
)OpenXml
There may be a more proper way to remove a GAC file (below), but that is what I did and it worked.
gacutil –u DocumentFormat.OpenXml.dll
Hope that helps!