A document has to do with the written material where information is written and stored for easy retrieval and can easily be marked to be separated from other documents.
As a result of this, we can see that a document can either be a primary source or a secondary source where the information contained in it were witnessed first hand by the writer or if he got his information from second hand sources.
A document which contains a primary source is generally more reliable than that of a secondary source because the writer saw the event happen live and the information has not been distorted by other sources.
Please note that your question is incomplete so I gave you a general overview so that you would better understand the concept.
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