10. The method according to claim 9 , wherein the API for manipulating the document is an API of an open document for opening the digital document, and wherein the document manipulation library is called via an X_Duplicate defined in the pre-processing of the API to be actually called, a copy of the digital document is generated, the document manipulation library is called via an X_OpenDocument defined in the main processing of the API to be actually called, the copied digital document is opened, the document manipulation library is called via an X_CallBackMessage defined in the post-processing of the API to be actually called, and the application program is notified that not the digital document but the copied digital document is generated and opened.
9. The method according to claim 8 , wherein, in a case where an API is not defined in the pre-processing or the post-processing of the API to be actually called, the document manipulation library is called via only an API of the main processing and digital document is manipulated.