Demonstrate how to check that a file can be downloaded

BookDownload_StepDefs.java

import cucumber.api.java.en.Given;
import cucumber.api.java.en.Then;
import cucumber.api.java.en.When;
import io.restassured.RestAssured;
import io.restassured.response.Response;
import org.openqa.selenium.By;
import org.openqa.selenium.Cookie;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;

import java.io.IOException;

import static org.hamcrest.Matchers.equalTo;
import static org.hamcrest.Matchers.greaterThan;
import static org.hamcrest.core.Is.is;
import static org.junit.Assert.assertThat;

public class BookDownload_StepDefs
{
	// STUB
	class BookStoreAPI {
		public BookStoreAPI login(String username, String password) {return this;}
		public BookStoreAPI addItem(String book) {return this;}
		public BookStoreAPI purchase(String paymentMethod) {return this;}
		public Response send() { return null; }
		public String getAuthToken() { return ""; }
	}

	BookStoreAPI bookstoreAPI = new BookStoreAPI();
	WebDriver driver = new ChromeDriver();
	Response response;
	String downloadURL;
	byte[] pdfFile;

	@Given("I have paid for my book")
	public void pay_for_book()
	{
		// USING A REST API TO AVOID SELENIUM STEPS
		response = bookstoreAPI.login("username", "password")
				.addItem("book")
				.purchase("paymentMethod")
				.send();
	}

	@When("I go to download page")
	public void go_to_download_page()
	{
		driver.manage().addCookie(new Cookie("authToken", bookstoreAPI.getAuthToken()));
		driver.get("downloadURL");
	}

	@When("I download the PDF version of the book")
	public void download_book() throws Exception
	{
		downloadURL = driver.findElement(By.id("downloadPDFLink")).getAttribute("href");
	}

	@Then("the file should be downloadable")
	public void check_download()
	{
		// USE A REST CLIENT TO CHECK THAT A FILE CAN BE DOWNLOADED
		response = RestAssured.head(downloadURL);

		assertThat(response.getStatusCode(), is(equalTo(200)));
		assertThat(response.getContentType(), is(equalTo("application/pdf")));
	}

	@Then("I should be able to read the PDF file")
	public void read_pdf() throws IOException
	{
		// USE A REST CLIENT TO ACTUALLY DOWNLOAD A FILE
		Response response = RestAssured.get(downloadURL);
		if (response.statusCode() != 200) { throw new RuntimeException("Book did not download"); }

		assertThat(response.getContentType(), is(equalTo("application/pdf")));

		pdfFile = response.asInputStream().readAllBytes();
		assertThat(pdfFile.length, is(greaterThan(0)));
	}
}

download_book.feature

Feature: Download book

Scenario:
    Given I have paid for my book
    When I go to the download page
    And I download the PDF version of the book
    Then the file should be downloadable
    And I should be able to read the PDF file

I don't think you need the "throws Exception" on line 53, do you?