profile = FirefoxProfile()
profile.set_preference('browser.helperApps.neverAsk.saveToDisk',
    'application/pdf')
driver = webdriver.Firefox(firefox_profile=profile)
driver.get(target_url)
driver.find_element_by_css_selector('a[title="Click to Download"]').click()
How do I automatically download files from a pop up dialog using selenium-python