2.2 PDF document structure
PDF document PDF document content structure is the logic of organizational structure. It reflects the document body object indirect relationship between the hierarchy. PDF of the document structure is a tree structure, in Figure 2. Tree root node is the root of the target PDF document. There are four under the root node subtree: tree pages (Pages tree), a bookmark tree (out line tree), lead tree (Articleth reads), the name of the tree (named Destination). One tree on the page, all page objects in the tree leaf nodes, the sub-tree nodes will inherit the parent node of the attribute values as the corresponding default value attributes. Bookmark tree hierarchy according to tree-level relations will bookmark (booKmarK) organized. Book signing set up a bookmark with a specific location on the page link, which allows users to bookmark name in accordance with the contents of the document to access. Bookmarks can be due to the level of documentation can be used to organize the directory, it will be a bookmark tree sometimes called the directory tree. The article will be clues to the tree and the trail leads under the block of the article (Article bead) organized by tree structure management. Article, a good block is a predefined region on the page, it is generally a section of readers interested in the text or images, which purpose is to allow the entire visual areas to show only the specific area to avoid interference with other parts of the page. The article leads to a predefined series pieces together a good article, if the article leads readers to read by, the browser only the sequence revealed that the clues in the pieces of the article so that readers read the content of their own interest, rather than by the order read. As for the name of the tree is to build a string (name), and pages of correspondence between the region, tree leaf nodes to preserve the string and the corresponding page region, rather than just a leaf node index to allow applications to quickly access to the leaf nodes. The name of the role of trees is to allow PDF documents to other objects in the string name used to represent the region on one page.
2.3 PDF resource
PDF of the page content (such as text, graphics, images) are stored in the page contents object stream object corresponding to the keyword (below referred to as the content streams) in. Content stream using a number of basic objects such as the number of strings, these are the direct object. But there are a number of other objects such as fonts, their object is to use a dictionary (Dictionary), or stream object (stream) to show that, the direct object can not be said that the contents of the stream can not be any indirect object (or unable to communicate with the content of their own to distinguish between data), so we named these objects, and in the content stream with the corresponding name indicated that they. These used to indicate the name of the object referred to as named on the resources (named resources).
Object on the page of a resource (resource skey), lists the contents of the flow of all resources used, and the establishment of a resource name and resource mapping table object itself.
PDF of the named resources are as follows: Instruction Set (Procset), fonts (font), color space (color space), the external object (xobJect ), the expansion of the graphics state (extended graphics state), at the end of pattern (Pattern), Extensible Markup list of users (Property list).
Non-naming resources: encoding, font Descriptor, halftone, fuction, ¡C CMAP. Resources as a result of non-naming were cited implied, there is no need to rename.
2.4 PDF page description commands
PDF of a page description of a total of 60 instructions. This page describes the 60 page instruction describes a series of graphic object. These graphical objects can be divided into four categories: the target path (Path object), the text of the object (text object), the image object (image object), the external object, see Figure 3. It constitutes an essential element of all the pages.