you can write the full path to a single file, instead of *.docx

$dollar$Files = Get-ChildItem "D:*.docx"

$dollar$Word = New-Object -ComObject Word.Application

Foreach ($dollar$File in $dollar$Files) {

  # open a Word document, filename from the directory

  $dollar$Doc = $dollar$Word.Documents.Open($dollar$File.FullName)

  # Swap out DOCX with PDF in the Filename

 $dollar$Name=($dollar$Doc.FullName).Replace("docx","pdf")

  # Save this File as a PDF in Word 2010/2013

  $dollar$Doc.SaveAs(oref] $dollar$Name, fref] 17)

  $dollar$Doc.Close()

}