What type of document is characterized by having no fixed format?

The term "unstructured documents" refers to documents that do not adhere to a pre-defined format or organization. These types of documents can include a wide variety of content, such as text files, emails, social media posts, and multimedia files, which typically contain information that is not organized in a structured way. This lack of a fixed format makes it challenging to extract specific data or insights from them without advanced processing techniques, such as natural language processing or machine learning.

In contrast, structured documents have a fixed format, which allows for easier data extraction and analysis. Semi-structured documents may have some organizational elements (like tags or markers) but lack a strictly defined format. Digital documents simply refer to any document that exists in digital form, which can include both structured and unstructured types. Therefore, the defining characteristic of unstructured documents is their absence of a fixed format, making them the correct answer in this context.