1. A document processing method, comprising the steps of: (a) obtaining a document, dividing the document into a plurality of terms, and giving a syntactic index to each of the terms; (b) dividing the document into a plurality of sentences according to at least one type of punctuation marks contained in the document; (c) selecting a plurality of key syntactic indexes from the syntactic indexes, transforming each of the sentences into a preliminary key terms sequence according to the key syntactic indexes, wherein each preliminary key terms sequence includes at least one preliminary key term corresponding to the key syntactic indexes; (d) selectively expanding the at least one preliminary key term of a representative sentence of the document into an advanced key term; (e) determining at least one core candidate sentence from the plurality of sentences according to the preliminary key term and the advanced key term to form a core abstract of the document; (f) forming a matrix by a key term of the preliminary key term and the advanced key term and the preliminary key terms sequence of the Step (c); (g) performing a singular value decomposition of the matrix to obtain a weight of the sentence corresponding to each preliminary key terms sequence; (h) selecting at least one of the sentences as an auxiliary candidate sentence according to the weight, and combining the at least one auxiliary candidate sentence into a supplementary abstract of the document; (i) receiving a compression ratio, and calculating the number of sentences in an abstract according to the compression ratio; (i) determining whether or not the number of sentences contained in the core abstract is smaller then the number of sentences of the abstract; (k) selecting the sentence from the core abstract having the number of sentences of the abstract as a displaying abstract of the document, if the determination of Step (i) is negative; and (l) selecting and merging the auxiliary candidate sentence which is not a repeat of the core abstract from the supplementary abstract into the core abstract to form the displaying abstract of the document if the determination of Step (i) is affirmative.