1. A document processing system comprising: a processor and a memory; a storage unit that stores a plurality of items of document data; a receiving unit that receives document data via a network; a scanning unit that scans a document and inputs document data of the document; an analyzing unit that analyzes the document data input by the scanning unit and extracts content data embedded in the input document data; a determining unit that determines whether the document data is received by the receiving unit or the document data is input by the scanning unit; a specifying unit that specifies, in a case where the determining unit determines that the document data is received by the receiving unit, based on meta data of the document data, related document data related to the document data received by the receiving unit among the plurality of items of document data stored in the storage unit, and specifies, in a case where the determining unit determines that the document data is input by the scanning unit, based on the content data extracted by the analyzing unit, related document data related to the document data input by the scanning unit among the plurality of items of document data stored in the storage unit; and a control unit that controls to store, in the storage unit in a case where the determining unit determines that the document data is received by the receiving unit, relation information representing a relation between the document data received by the receiving unit and the related document data specified by the specifying unit, and controls to store, in the storage unit in a case where the determining unit determines that the document data is input by the scanning unit, relation information representing a relation between the document data input by the scanning unit and the related document data specified by the specifying unit, wherein at least one of the storage unit, the receiving unit, the scanning unit, the analyzing unit, the specifying unit and the control unit is implemented by the processor and the memory.