1. A display control apparatus for a composite document processing apparatus, comprising: document data storage means for separately storing different types of data, including character code data, image data and graphic data, which together constitutes a composite document; data attribute storage means for storing attribute data for each data stored in said document data storage means, said attribute data including a location in said composite document, an area size and a type of said each data stored in said document data storage means; window/viewport storage means for storing attribute data including a window size and a viewport size; display data storage means for storing image-formation display data; display means for displaying said image-formation display data; and control means for writing said data stored in said document data storage means into said display data storage means in an image form in accordance with said attribute data stored in said data attribute storage means and said attribute data stored in said window/viewport storage means, said control means including: first display control means for reducing all of said composite document or an arbitrary portion of said composite document responsive to a size of a first window, indicating a region of said document data storage means where data is to be displayed, and a first viewport for indicating a display region of said display means on which said display data on said first window is to be displayed, said reducing being such that said first window falls within said first viewport, and for writing said reduced document data into said display data storage means in an image form, thus providing a reduced display to said display means when said reduced document data is read therefrom, second display control means for performing an equal enlargement or a greater of a second window, which has as a maximum size the same size as said first window, and a display data within said second window, in such a manner that said second window falls within a second viewport corresponding to said second window, and for writing said enlarged display data in said display data storage means in an image form, thereby providing an enlarged display to said display means, and third display control means, responsive to an updating of data by said second display control means for updating, in real time, data displayed on said first viewport which corresponds to updated data.