PrintDocumentToFile - file will not release

I am helpless :) One function of my Application is to convert PDF Files to Postscript by using the PrintDocumentToFile function.
So far so good, but the file will not write to the desired destination, while my application is running.
It is shown in the windows spooler and will release if my application is stopped.

I call the printer class, that uses the QuickPdf Library, within a using-Block and in addition i use a Garbage Collect after that.

So i can't explain it to me, why the file will be released before the application stopped.
May be there is a closure missing, like CloseDoc to release the Document?

Here is a short version of my code:
private void PrintToFile()
{
   var cp = objPDF.NewCustomPrinter(_PrinterName);
   objPDF.SetupCustomPrinter(cp, 5, 1);           // Set Quality

   var iPrintOptions = objPDF.PrintOptions(_PageScaling, 1, Path.GetFileName(_PrintFileName));

   objPDF.PrintDocumentToFile(cp, _StartPage, _EndPage, iPrintOptions, _OutputFile);
}

Does anyone have any idea why this is so?

Thanks Lutz.