Documentary evidence is most widely understood to refer to writings on paper (such as an invoice, a contract or a will), but the term can also apply to any media by which information can be preserved, such as photographs; a medium that needs a mechanical device to be viewed, such as a tape recording or film; and a printed form of digital evidence, such as emails or spreadsheets.
For instance, when original is lost or is in custody or possession of person against whom it is sought to be proved.
However, a film of the murder taking place would be documentary evidence (just as a written description of the event from an eyewitness).
If the content of that same letter is then introduced to show the motive for the murder, then the evidence would be both physical and documentary.
Documentary evidence is subject to specific forms of authentication, usually through the testimony of an eyewitness to the execution of the document, or to the testimony of a witness able to identify the handwriting of the purported author.