FillPDF 8.x-4.0 is a true Drupal 8 port. FillPDF now supports all types of entities, uses plugins for handling actions and backends, and makes full use of object-oriented-style programming. All core features from Drupal 7 are supported. Some, such as Rules integration and Ubercart integration, remain to be ported (and in the case of the latter, will probably not be ported).
Documentation is coming soon. Post an issue if you actually want to use it on your site and need documentation.
Changes since 8.x-4.0-alpha1: