To convert a docx document to pdf the Word.Application Com Object can be used, for PowerShell Magazine I wrote a tip in which I describe this process. The full article  is available on PowerShell Magazine : Convert .docx to .pdf using Word.Application

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$Word = New-Object -ComObject "Word.Application"
($Word.Documents.Open('c:\temp\file.docx')).SaveAs([ref]'c:\temp\file.pdf',[ref]17) 
$Word.Application.ActiveDocument.Close()

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