FixedDocument logically binds an ordered sequence of pages together into a single, multi-page, fixed-layout document.
PageContent is the only allowable child element of the FixedDocument element. Each PageContent element refers to the source of the content for a single page. PageContent elements must be in sequential markup order, matching the page order of the document.
FixedDocument is designed for "what you see is what you get" (WYSIWYG) applications where the document layout is defined and controlled by the application for rendering to the highest accuracy of the display or print device.
FlowDocument, on the other hand, is designed to maximize user readability, and dynamically adjusts the document layout based on user preferences (for example, a user-chosen type size or font) and other variables, such as display size and resolution.