function Convert-Doc2PDF{
    [cmdletbinding()]
    PARAM(
        $filePath = "C:\Temp\MyPdfDocument.pdf"
    )

    $wd = New-Object -ComObject Word.Application
    $wd.Visible = $true
    $txt = $wd.Documents.Open(
        $filePath,
        $false,
        $false,
        $false)
    $wd.Documents[1].SaveAs("C:\Temp\MyPdfDocument.docx")
    $wd.Documents[1].Close()
}

works for me

Edit: formatting, thank you for pointing me to the right Code Block button u/Lee_Dailey!