<p>I am sure there is a simple answer to this one.</p>

<p>I have a docx file that I get an error when trying to download(document cannot be found).... .doc is fine .txt is also fine. I am sure this is just an iis setting, the permissions on the server are all the same for all files. </p>

When there is no mime type associated with an extension in IIS (6.0 and up), it will give you 404. See
Requests for static files return 404 error (IIS 6.0)
. (
Edit:
The original MSDN link is dead as of mid-2017,
the Internet Archive has a copy
. Even though it was written for IIS6, the information still applies to modern IIS versions.)
Other than that, and just because you did not specify
where
the error shows up: When you forbid caching in a header, similar behavior may occur on the client. But that is a general issue, and not bound to .docx.