Show how disabling the PDF viewer does not work. Tested in Chromium Version 60.0.3112.78 (Developer Build) (64-bit)

download_pdf_test.py

#!/usr/bin/env python

from selenium import webdriver
from selenium.webdriver.common.keys import Keys


def get_browser():
    """
    Get a chrome browser with disabled PDF viewer.

    See https://stackoverflow.com/a/41891043/562769
    """
    path_to_chromedriver = '/usr/lib/chromium-browser/chromedriver'
    chrome_profile = webdriver.ChromeOptions()
    profile = {"plugins.plugins_list": [{"enabled": False,
                                         "name": "Chrome PDF Viewer"}]}
    chrome_profile.add_experimental_option("prefs", profile)
    browser = webdriver.Chrome(executable_path=path_to_chromedriver,
                               chrome_options=chrome_profile)
    return browser


def download_file_example(browser):
    browser.get("https://martin-thoma.com/author/martin-thoma/")
    links = browser.find_elements_by_xpath('//a')
    for link in links:
        if link.get_attribute('href').endswith('.pdf'):
            print(link.get_attribute("outerHTML"))
            link.click()
            break

browser = get_browser()
download_file_example(browser)