Actually, there is no huge difference and few minute things bring about a thin line of difference.
The dashboard is more like a movie is used for visual representation where we use widgets, graphs and other functionalities.
The document is like a book which has a limitation where my data is restricted to grids and in achieving complex business. It has
Page headers, Document headers, Detail headers, Detail, Page footer, and Document header
A MicroStrategy document displays data coming from multiple reports, with the data laid out and designed in presentation-quality format. Most data on a document is from one or more underlying datasets. A dataset is a standard MicroStrategy report. Other document components that do not originate from the dataset, such as static text used for a title or heading, page numbers, and images, are added by the document's designer and are stored in the document's definition.
A dashboard is a special type of document. A dashboard is commonly only one page long, is intended to be viewed online, and usually provides interactive features that let analysts change how they view the dashboard’s data, as well as what data they are viewing.