1. A method for extracting a raster image of a page from a portable electronic document, comprising: (a) acquiring commands and resources of the raster image of the page by analyzing a format of the portable electronic document; (b) extracting first and second candidate raster images by processing the commands and the resources of the raster image of the page; (c) integrating the first and second candidate raster images as an integrated candidate raster image provided that the first and second candidate raster images are linked together; and (d) removing a pseudo-raster image from the integrated candidate raster image, wherein the step (b) includes: (b1) specifying an image display command in the acquired commands of the raster image of the page in the step (a); (b2) extracting positional and size information on the first and second candidate raster images by processing the commands of the raster image of the page based on the specified image display command in the step (b1); (b3) decrypting the first and second candidate raster images based on a decryption system specified for the portable electronic document; and (b4) storing the decrypted first and second candidate raster images in a first candidate raster image list.