Drawing.vsd has 3 pages. But below code only can get one page. If I save vsd as vdx, below code can get all pages.
// The path to the documents directory.
String dataDir = Utils.getDataDir(ExportToHTML.class);
// Call the diagram constructor to load diagram from a VSD file
Diagram diagram = new Diagram(dataDir+ "drawing.vsd");
PageCollection pages = diagram.getPages();