Automator command line: "Unable to open workflow"
Use -i:

automator -i foo.docx ~/Documents/convert_word_to_pdf.app


Or if there are multiple arguments:

printf %s\\n foo.docx bar.docx|automator -i - ~/Documents/convert_word_to_pdf.app


See the man page:

SYNOPSIS
     automator [-v] [-i input] [-D name=value ...] workflow

DESCRIPTION
     automator runs the specified workflow.  To create or edit a workflow, use
     the Automator application.

     The following options are available:

     -D name=value
              Set variable name to value for this run of workflow.

     -i input
              Set input as the input to workflow If input is - then the con-
              tents of standard input is used.  The newline character (\n) is
              the delimiter for multiple strings.

     -v       Run in verbose mode.