Как я могу найти элемент классом CSS с XPath?

Вы должны использовать StreamingOutput вместо использования File.

String homePath = System.getProperty("user.home");
    String filePath = homePath + "/Downloads/Resume" + LocalDateTime.now().toLocalDate() + ".pdf";

    org.xhtmlrenderer.pdf.ITextRenderer renderer = new ITextRenderer();

    String yourXhtmlContentAsString = "<h1>hi </h1>";
    renderer.setDocumentFromString(yourXhtmlContentAsString);
    renderer.layout();

    java.io.FileOutputStream fos = new java.io.FileOutputStream(filePath);
    renderer.createPDF(fos);
    //fos.close();
    final File file = new File(filePath);

    StreamingOutput fileStream =  new StreamingOutput()
    {
        @Override
        public void write(java.io.OutputStream output) throws IOException, WebApplicationException
        {
            try
            {

                byte[] data = Files.readAllBytes(file.toPath());
                output.write(data);
                output.flush();
            }
            catch (Exception e)
            {
                throw new WebApplicationException("File Not Found. !!");
            }
        }
    };

   return Response
       .ok( fileStream, MediaType.APPLICATION_OCTET_STREAM)
       .header("Content-Disposition", "attachment; filename=\"Resume" + LocalDateTime.now().toLocalDate() + ".pdf\"")
       .build();