What is the correct way to use a Symfony bundle? I want to use https://github.com/KnpLabs/KnpSnappyBundle in a Drupal 8 project. How do I correctly use it?
Drupal 8 isn't using Symfony bundles, only some Symfony components. Therefore you should use https://github.com/KnpLabs/snappy instead of bundle you've mentioned. You should install it using composer and then use available classes in your Drupal code. If you want to create PDF-generation service, you should follow official Drupal documentation.