Probleme mit Powershell skript und REF

$documents_path = 'c:\pdftest'  

$word_app = New-Object -ComObject Word.Application

# This filter will find .rtf documents
Get-ChildItem -Path $documents_path -Filter *.rtf? | ForEach-Object {

    $document = $word_app.Documents.Open($_.FullName)

    $pdf_filename = "$($_.DirectoryName)\$($_.BaseName).pdf"  

    $document.SaveAs([ref] $pdf_filename, [ref] 17)

    $document.Close()
}

$word_app.Quit()

Argument: "1" darf nicht System.Management.Automation.PSReference sein. [ref] darf nicht verwendet werden.  
Bei C:\pdftest\pdfrtf.ps1:12 Zeichen:21
+     $document.SaveAs <<<< ([ref] $pdf_filename, [ref] 17)
    + CategoryInfo          : NotSpecified: (:) , MethodException
    + FullyQualifiedErrorId : RefArgumentToNonRefParameterMsg

Name                           Value
----                           -----
CLRVersion                     2.0.50727.4952
BuildVersion                   6.1.7600.16385
PSVersion                      2.0
WSManStackVersion              2.0
PSCompatibleVersions           {1.0, 2.0}
SerializationVersion           1.1.0.1
PSRemotingProtocolVersion      2.1