Thanks for your inquiry. I am afraid it is not possible to restrict download of a document using Aspose API. As API is class library and does nothing with GUI/Viewer, Save/Save As options are not a feature / property of PDF document but these are the features of the Tool / Viewer which is used to display the PDF document and Aspose.Pdf can only manipulate the PDF document and not the PDF Viewer /Tools.
However in order to view/display PDF files, you may consider using ViewerApp of our sister company named GroupDocs, as this application provides the feature to restrict PDF downloading. Please note that our EULA is displayed using ViewerApp and as you can notice, there is no save option to download the file within viewer.