2. A document extracting apparatus comprising: a processing device; a storage device storing instructions that, when executed by the processing device, cause the processing device to implement: storing a document index, the document index including a page index having page indices respectively indicating pages of the document data and a number of pages for each document in association with document data corresponding to each page included in the document; storing feature data, calculated based on a feature point extracted from the document data and indicative of a feature of the document data, in association with the document data; obtaining input document data serving as new document data; extracting a feature point from the input document data; generating feature data indicative of a feature of the input document data based on the extracted feature point; comparing the generated feature data with the stored feature data, thereby judging similarity between the document data associated with the feature data and the input document data; obtaining a document index associated with document data that is judged as document data highly similar to the input document data; and extracting a plurality of pieces of document data corresponding to a plurality of pages included in a document indicated by the obtained document index.
2. A document extracting apparatus comprising: a processing device; a storage device storing instructions that, when executed by the processing device, cause the processing device to implement: storing a document index, the document index including a page index having page indices respectively indicating pages of the document data and a number of pages for each document in association with document data corresponding to each page included in the document; storing feature data, calculated based on a feature point extracted from the document data and indicative of a feature of the document data, in association with the document data; obtaining input document data serving as new document data; extracting a feature point from the input document data; generating feature data indicative of a feature of the input document data based on the extracted feature point; comparing the generated feature data with the stored feature data, thereby judging similarity between the document data associated with the feature data and the input document data; obtaining a document index associated with document data that is judged as document data highly similar to the input document data; and extracting a plurality of pieces of document data corresponding to a plurality of pages included in a document indicated by the obtained document index. 4. The document extracting apparatus according to claim 2 , wherein the document data obtaining step obtains a plurality of pieces of input document data, wherein the judgment step judges, for each of a plurality of pieces of the input document data, similarity between the stored document data and the input document data, and wherein when document index associated with document data highly similar to each of a plurality of pieces of the input document data is identical to each other, the document data extracting step extracts a plurality of pieces of document data corresponding to a plurality of pages included in a document indicated by the document index.