Use the PdfViewer.ZoomFactor property instead to set the document zoom factor.
System.Object
Command
DevExpress.Utils.Commands.ControlCommand<DevExpress.XtraPdfViewer.PdfViewer, DevExpress.XtraPdfViewer.Commands.PdfViewerCommandId, DevExpress.XtraPdfViewer.Localization.XtraPdfViewerStringId>
PdfViewerCommand
PdfSetZoomCommand