You can try this code :-I think this is what you are looking for. let me know If this what you are expecting.
System.setProperty("webdriver.gecko.driver", "D:/geckodriver.exe");
WebDriver driver = new FirefoxDriver();
driver.manage().window().maximize();
driver.get("http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/pdf_open_parameters.pdf");
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
Thread.sleep(2000);
java.awt.Robot robot = new java.awt.Robot();
Thread.sleep(1000);
robot.keyPress(KeyEvent.VK_CONTROL);
robot.keyPress(KeyEvent.VK_S);
robot.keyRelease(KeyEvent.VK_S);
robot.keyRelease(KeyEvent.VK_CONTROL);
Thread.sleep(2000);
robot.keyPress(KeyEvent.VK_ENTER);
Thread.sleep(2000);
robot.keyPress(KeyEvent.VK_TAB); // file replace move to yes button
Thread.sleep(2000);
robot.keyPress(KeyEvent.VK_ENTER); // hit enter
Just first execute the code, see if it is working, and is it what you want.
Last three lines of code are written for replace existing pdf file. So you just first comment those three lines, execute the code and from next time, include last three lines of code
You need to use Robot Class to handle events.
And let me know whether this is working at your end.