If you run specs or your application and get an error like:
ActionController::MissingFile in 'ProductsController#show, should render PDF'
Cannot read file /some/file.pdf

Rails Long Term Support

Rails LTS provides security patches for old versions of Ruby on Rails (2.3, 3.2, 4.2 and 5.2)

• Prevents you from data breaches and liability risks
• Upgrade at your own pace
• Works with modern Rubies

Read more Show archive.org snapshot

You may be missing the HTMLDOC Show archive.org snapshot binary on your system. Install it like this on Debian/Ubuntu:
sudo apt-get install htmldoc