Login to WebClient -> Settings -> XML publish plugin settings
Search for PDF
Find the pdf definitions. It should look something like this
<plugindefinition name="ISHFINDDITAOTPDFRESULTFILE" handler="IshFindFile">
<description>Finds the file in the DITA-OT output folder based on a certain file pattern matching rule.</description>
<initialize>
<parameters>
<parameter name="FolderContextItemsKey">DITAOTOutDirectory</parameter>
<parameter name="FilePattern">*.pdf</parameter>
<parameter name="FoundFileContextItemsKey">DITAOTOutputFile</parameter>
</parameters>
</initialize>
</plugindefinition>
Update the plugin definition name to something unique (ISHFINDDITAOTWORDRESULTFILE)
Change the file pattern to *.docx. The file should look like this now
<parameter name="FilePattern">*.docx</parameter>