In the latest years and especially after the global COVID19 pandemic, digital learning environments have become an important educational aid all around the world. In light of this, our Physics Education Research group has developed a new digital learning format called a “Digital Learning Module” (DLM), consisting of a series of pre-recorded explanatory and experiment videos, interactive elements, an interactive simulation experiment, and expert interviews. So far, our group has created one DLM on the topic of Positron Emission Tomography (PET). By integrating different theoretical and empirical studies, this research seeks to identify and evaluate principles fitting the format of the DLM. The evaluation process involved first an analysis of the PET DLM in terms of the identified design principles, then 9 semi-structured interviews with high-school students from Greece and Austria as they worked through the PET DLM. A Qualitative Content Analysis of the interview transcripts has determined the set of effective design principles that will constitute the foundation of future DLMs developed by our group and can serve as starting point and reference to researchers developing similar digital learning environments elsewhere.