iText 7 페이지 번호 넣기

int n = pdf.getNumberOfPages();
Paragraph footer;
for (int page = 1; page <= n; page++) {
    footer = new Paragraph(String.format("Page %s of %s", page, n));
    document.showTextAligned(footer, 297.5f, 20, page,
        TextAlignment.CENTER, VerticalAlignment.MIDDLE, 0);
}
document.close();

java.lang.NullPointerException
	at com.itextpdf.kernel.pdf.PdfDictionary.get(PdfDictionary.java:552)
	at com.itextpdf.kernel.pdf.PdfDictionary.get(PdfDictionary.java:146)
	at com.itextpdf.kernel.pdf.PdfPage.getContentStreamCount(PdfPage.java:203)
	at com.itextpdf.kernel.pdf.PdfPage.getLastContentStream(PdfPage.java:230)
	at com.itextpdf.kernel.pdf.canvas.PdfCanvas.getPageStream(PdfCanvas.java:2375)
	at com.itextpdf.kernel.pdf.canvas.PdfCanvas.<init>(PdfCanvas.java:236)
	at com.itextpdf.layout.renderer.DocumentRenderer.flushSingleRenderer(DocumentRenderer.java:118)
	at com.itextpdf.layout.renderer.RootRenderer.addChild(RootRenderer.java:225)
	at com.itextpdf.layout.RootElement.add(RootElement.java:108)
	at com.itextpdf.layout.Document.add(Document.java:143)
	at com.itextpdf.layout.Document.add(Document.java:68)
	at com.itextpdf.layout.RootElement.showTextAligned(RootElement.java:359)

document = new Document(pdf, PageSize.A4);

document = new Document(pdf, PageSize.A4, false);