/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {
    File file = new File("");

    PDDocument doc = PDDocument.load(file);
    PDFRenderer pdfRenderer = new PDFRenderer(doc);
    BufferedImage image = pdfRenderer.renderImageWithDPI(0, 300);

    ImageIOUtil.writeImage(image,"", 300);
    doc.close();
}

/**
 * @param args the command line arguments
 * @throws java.io.IOException
 */
public static void main(String[] args) throws IOException {
    File file = new File("");

    PDDocument doc = PDDocument.load(file);
    PDFRenderer pdfRenderer = new PDFRenderer(doc);
    BufferedImage image = pdfRenderer.renderImageWithDPI(0, 300);

    ImageIOUtil.writeImage(image,"", 300);
}

Opens a PDF Document and renders the first page as an image, which is then saved to file.

PDFBox: Render first page in a document

Java