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. 3. The document extracting apparatus according to claim 2 , storing, in association with one piece of document data, a plurality of pieces of feature data indicative of a feature of the document data, wherein the generation step generates a plurality of pieces of feature data indicative of a feature of input document data, and wherein the judgment step includes: voting, for each of a plurality of pieces of the feature data generated by the generation step, for document data associated with feature data identical to the feature data; and deciding, as document data highly similar to the input document data, document data having the greatest number of votes or document data having the number of votes equal to or greater than a predetermined amount, among the stored document data.