1. A digital document processing apparatus comprising: a digital document storage unit configured to store a digital document; and a program storage unit configured to store an application program used for controlling the digital document, running on an application platform, an application programming interface (API) for manipulating the digital document called from the application program, a document manipulation library for manipulating the digital document called via the API, and a filter plug-in defined between the API and the document manipulation library; wherein the filter plug-in is connected to a filter, and wherein the connected filter contains a mapping table in which rules for changing the API called by the application program to an API to be actually called; and a control unit configured to cause the filter plug-in connected to a filter to acquire the API for manipulating the document called by the application program, cause the filter plug-in to change the API for manipulating the document to the API to be actually called based on the mapping table in the filter, call the document manipulation library via the changed API to be actually called, and manipulate the digital document.
1. A digital document processing apparatus comprising: a digital document storage unit configured to store a digital document; and a program storage unit configured to store an application program used for controlling the digital document, running on an application platform, an application programming interface (API) for manipulating the digital document called from the application program, a document manipulation library for manipulating the digital document called via the API, and a filter plug-in defined between the API and the document manipulation library; wherein the filter plug-in is connected to a filter, and wherein the connected filter contains a mapping table in which rules for changing the API called by the application program to an API to be actually called; and a control unit configured to cause the filter plug-in connected to a filter to acquire the API for manipulating the document called by the application program, cause the filter plug-in to change the API for manipulating the document to the API to be actually called based on the mapping table in the filter, call the document manipulation library via the changed API to be actually called, and manipulate the digital document. 6. The digital document processing apparatus according to claim 1 , further comprising: an authentication unit configured to authenticate and identify a user; and a determination unit configured to determine a filter to connect based on authentication information transmitted from the authentication unit.