1. A document processing system comprising: a unique information acquiring section that acquires unique information to be recorded; a similar document element selecting section that selects one similar document element corresponding to the unique information from each of one or a plurality of groups of similar document elements which relate to one or a plurality of document elements respectively, the one or the plurality of document elements contained in document data; a replacement document data acquiring section that acquires replacement document data which is generated by replacing the document elements with the respective similar document elements selected by the similar document element selecting section; a position information acquiring section that acquires a piece or pieces of position information indicating a position or positions of the one or the plurality of document elements contained in the document data; and a registering section that registers the groups of similar documents elements in a storage unit in such a manner that the pieces of position information acquired by the position information acquiring section are correlated with the document elements respectively, wherein the one or the plurality of document elements are selected by a user, and each of the one or the plurality of document elements in a word contained in the document data, wherein the registering section registers the groups of similar document elements in such a manner that similar document element numbers starting from 0 are correlated, in number order, with the respective similar document elements of each group, and where the registering section registers the document elements in number order as a table form with weights correlated with the respective document elements in such a manner that a weight corresponding to a first document element is 1 and a weight corresponding to an Nth document element is obtained by multiplying a weight of an (N −1)th document element by a number of similar document elements of a group of similar document elements which relates to the (N−1)th document element, when N is an integer that is larger than or equal to 2.