PDFBox を使って PDF ファイルを画像に変換するにはどうすればよいですか?

PDDocument document = PDDocument.loadNonSeq(new File(pdfFilename), null);
List<PDPage> pdPages = document.getDocumentCatalog().getAllPages();
int page = 0;
for (PDPage pdPage : pdPages)
{
    ++page;
    BufferedImage bim = pdPage.convertToImage(BufferedImage.TYPE_INT_RGB, 300);
    ImageIOUtil.writeImage(bim, pdfFilename + "-" + page + ".png", 300);
}
document.close();

PDDocument document = PDDocument.load(new File(pdfFilename));
PDFRenderer pdfRenderer = new PDFRenderer(document);
for (int page = 0; page < document.getNumberOfPages(); ++page)
{
    BufferedImage bim = pdfRenderer.renderImageWithDPI(page, 300, ImageType.RGB);
    ImageIOUtil.writeImage(bim, pdfFilename + "-" + (page+1) + ".png", 300);
}
document.close();

PDDocument document = Loader.loadPDF(new File(pdfFilename));