코드

    public static void main(String[] args) throws IOException {
        Path path = Paths.get("pdf.jpg");
        try (InputStream in = new FileInputStream("pdf.pdf");
                OutputStream out = Files.newOutputStream(path);) {
            rasterize(in, out);
        }

        System.out.println(path.toAbsolutePath());
    }

    public static void rasterize(InputStream in, OutputStream out) throws IOException {
        try (PDDocument doc = PDDocument.load(in)) {

            PDFRenderer pdfRenderer = new PDFRenderer(doc);

            BufferedImage bim = pdfRenderer.renderImageWithDPI(0/*1ページ目を対象にする*/, 300, ImageType.RGB);

            ImageIO.write(bim, "JPEG", out);
        }
    }

결과