10. A non-transitory computer-readable medium embodying a set of instructions which, when executed by a computer, cause the computer to: process document images into one or more documents, wherein a document of the one or more documents includes multiple pages; maintain a page-based coordinate system to specify a location of structures within individual pages of the document; combine the multiple pages to form a multi-page sheet, wherein a sheet-based coordinate system specifies a location of structures within the multi-page sheet; and perform, by the computing device, a data extraction operation to extract data from the document, said data extraction operation including: detecting the structures on the individual pages using the page-based coordinate system; defining a repeating group of fields, wherein the repeating group of fields is capable of flowing over from one page onto another page; detecting whether all fields of an instance of the repeating group of fields are found on consecutive pages of the document; and depending on whether all fields of the instance of the repeating group of fields are found on consecutive pages, detecting structures within the document using the sheet-based coordinate system.
10. A non-transitory computer-readable medium embodying a set of instructions which, when executed by a computer, cause the computer to: process document images into one or more documents, wherein a document of the one or more documents includes multiple pages; maintain a page-based coordinate system to specify a location of structures within individual pages of the document; combine the multiple pages to form a multi-page sheet, wherein a sheet-based coordinate system specifies a location of structures within the multi-page sheet; and perform, by the computing device, a data extraction operation to extract data from the document, said data extraction operation including: detecting the structures on the individual pages using the page-based coordinate system; defining a repeating group of fields, wherein the repeating group of fields is capable of flowing over from one page onto another page; detecting whether all fields of an instance of the repeating group of fields are found on consecutive pages of the document; and depending on whether all fields of the instance of the repeating group of fields are found on consecutive pages, detecting structures within the document using the sheet-based coordinate system. 15. The non-transitory computer-readable medium of claim 10 , wherein a flexible structure description for the document includes descriptions of data fields to be detected and descriptions of anchor elements and their relationships within documents of a given type.