highlightword.ps1

$objWord = New-Object -ComObject word.application
$objWord.Visible = $True
$objDoc = $objWord.Documents.Open("C:\temp\test.docx")


$FindText = "document"

  foreach ($docrange in $objDoc.Words)
  {
     if ($docrange.Text.Trim() -eq $FindText)
     {
        $docrange.highlightColorIndex = [Microsoft.Office.Interop.Word.WdColorIndex]::wdYellow 
     }
  }