I know you can do it with Apple script and Pages, and I’m going to figure out that tool at some point. But for now, I personally do it with this little shell script which I shamelessly stole from somewhere else, so I left the author info in there:
#!/bin/bash
# Jacob Salmela
# 2016-03-12
# Convert annoying DOCX into PDFs with a right-click
# Run this as an Automator Service
###### SCRIPT #######
for f in "$@"
do
# Get the full file PATH without the extension
filepathWithoutExtension="${f%.*}"
# Convert the DOCX to HTML, which cupsfilter knows how to turn into a PDF
textutil -convert html -output "$filepathWithoutExtension.html" "$f"
retVal=$?
if [ $retVal -ne 0 ]; then
exit $retVal
fi
# Convert the file into a PDF
cupsfilter "$filepathWithoutExtension.html" > "$filepathWithoutExtension.pdf"
retVal=$?
if [ $retVal -ne 0 ]; then
exit $retVal
fi
# Remove the original file and the temporary HTML file, leaving only the PDF
rm "$f" "$filepathWithoutExtension.html" >/dev/null
done