1. A computer-implemented method for processing document data, executed by one or more processors by one or more computer-readable instructions when read, wherein the document data comprises at least one page in order and the each page comprising at least one layer and one layer is overlaid with another layer, the method comprises: describing visible contents of each layer of the page in at least one layout object by utilizing a universal document model that is independent of formats of the document data; embedding a source file in an application's own format as a source file object into the layer; wherein, when editing document data in a layer of a page, the method further comprises: extracting the source file from the source file object corresponding to the layer and providing the source file for the application to edit; updating the at least one layout object of the layer based on the edited source file so that the visible contents of the page reflect edits by the application; re-embedding the edited source file into the source file object of the layer of the page; receiving an instruction indicating getting a page bitmap of at least one layer; generating a page bitmap, wherein all layout objects in the at least one layer are displayed in order in the page bitmap; and returning the page bitmap for displaying.