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If you are enrolled in a Goethe-Institut course or a university German program, your teacher might already have a license to share the PDF via the school’s closed network (e.g., Moodle or Blackboard). Ask your instructor. If they have a digital desk copy, they are legally allowed to share it with their students.
Sometimes, publishers offer free samples or resources for teachers and learners. You can check the Cornelsen website or contact their customer service to see if they offer any free resources or trials for "Grammatik Aktiv A1-B1."