Explainpaper (https://www.explainpaper.com/) explains the difficult passages of a pdf file.
Research oriented but works with any pdf.

After importing a pdf document (import or url), Explainpaper provides an explanation of the highlighted passage with the possibility of linked questions and answers.
6 levels of explanation proposed.

Zotero library import supported.

Free and unlimited highlight explanations and follow-up questions.
Other features in paid version.

Source for the tests: Artificial intelligence and education: research contributions and challenges for public policy