8. The method according to claim 2 , wherein the API to be actually called includes pre-processing, main processing and post-processing, and wherein the document manipulation library is called via at least one of APIs of the three kinds of processing according to definition of the API to be actually called, and the digital document is manipulated.
8. The method according to claim 2 , wherein the API to be actually called includes pre-processing, main processing and post-processing, and wherein the document manipulation library is called via at least one of APIs of the three kinds of processing according to definition of the API to be actually called, and the digital document is manipulated. 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.