Writing to a pdf document

How do I modify this method to write the contents of the modified document to a file name pdfOutput.  My aim is to get a document showing the modified changes so I can test the output:

    public static void addPageFooter(ByteArrayOutputStream baos) throws Exception {
        PdfReader reader = new PdfReader(baos.toByteArray());
        PdfStamper stamper = new PdfStamper(reader, baos);
        int numOfPages = reader.getNumberOfPages();
        for (int i=1; i<=numOfPages; i++)
            ColumnText.showTextAligned(
                    stamper.getUnderContent(i),
                    PdfContentByte.ALIGN_CENTER,
                    new Phrase("Page "+i),
                    reader.getPageSize(i).getWidth()/2,
                    5,
                    0);
        stamper.close();
        reader.close();
    }

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