The document zoom factor of an ASPxDocumentViewer is controlled by the XtraReport.PrintingSystem.Document.ScaleFactor property and depends on the ScaleFactor value saved into the HiddenField.
To manipulate the HiddenField value, use a custom editor added to the Document Viewer Toolbar (ReportToolbarComboBox).
Because the Document.ScaleFactor property value has no effect upon the actual document page size, it may be required to manually adjust the page size to match the current scale factor value.