1. A computer-implemented method for identifying PDF document, wherein, comprising the following steps: S 1 : analyzing the path objects in the PDF document, and identifying the table in the PDF document; S 2 : analyzing the text objects outside the identified table in the PDF document, and recognizing the text content in the PDF document; and S 3 : writing the identified results into a temporary file, or writing them into the PDF document as an attachment, wherein said step S 1 comprising: S 11 : extracting the line path objects in horizontal direction and vertical direction, recognizing one or multiple tables constituted by said line path objects according to the intersecting situations of said line path objects, wherein the process of extracting and recognizing include: taking each line segment of each line path object as a quadrilateral with 4vertexes, obtaining coordinates x1, y1, x2, y2of any two vertices in the same diagonal, and generating a rectangle by using the coordinates x1, y1, x2, y2as two vertices coordinates in the same diagonal of the rectangle; comparing an area difference between the quadrilateral and the rectangle, and if the area difference is greater than a threshold, then discarding the line segment; selecting rectangles satisfying a height-width ratio, and discarding the other rectangles; taking areas which are surrounded by line segments intersectant with each other as a potential form unit, storing the potential form unit in a data structure; overshadowing all the rectangles in each potential form unit to an axis which is parallel to a shorter line segment, combining reduplicated projection segments, and storing the reduplicated projection segments in the data structure; and taking a line which is vertical to the projection segment and beginning from a midpoint of the projection segment as split lines of rows and columns, storing all the split lines in the data structure; S 12 : finding out the maximum value and the minimum value of the extracted line endpoints in the horizontal direction coordinate and vertical direction coordinate, and defining the rectangular region of the table by those four values; and S 13 : analyzing the other objects in the rectangular region positioned in the table, wherein said step S 2 comprising: S 21 : finding the minimum value x Lmin of the text objects and the page left margin distances in the document; and S 22 : finding out the rows of which the distance of the start characters and the page left margin is x Lmin + fragment shrink, and be defined as the start rows; let the content from start row of each fragment to the start row of a next fragment is analyzed as one fragment.