The Github page defines TerminusDB as also a "document store", does it mean it can work like MongoDB storing JSON documents?
If so, to what extent it can be used to stored documents for user with web application data?
You can store documents in JSON-LD format in TerminusDB as long as the format follows the defined schema. The schema definitions constrain the allowed documents, and allow you to seamlessly move between a graph and document representation. They also allow you to perform quality control on the documents to make sure that required fields exist, and that these fields have the appropriate data-types.