If you're not using the upload feature in WebDocumentViewer, you can completely disable it in the following way

ASP.NET

In your WebDocumentRequestHandler.ashx codebehind, add a handler for FileUpload and in that handler, set the e.Cancel = true

C#

public WebDocViewerHandler()
{
    this.FileUpload += new FileUploadEventHandler(delegate(object sender, FileUploadEventArgs e) { e.Cancel = true; });
}

VB

Public Sub New()
     AddHandler Me.FileUpload, AddressOf fileUploadHandler
End Sub

Private Sub fileUploadHandler(sender As Object, e As FileUploadEventArgs)
    e.Cancel = True
End Sub

ASP.NET Core (targeting .NET Framework)

c#

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
        {
            loggerFactory.AddConsole();

            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseDefaultFiles();
            app.UseStaticFiles();

            RegisteredDecoders.Decoders.Add(new PdfDecoder() { Resolution = 200 });

            app.Map("/wdv", wdvApp => { wdvApp.RunWebDocumentViewerMiddleware(new MyWDVCallbacks()); });

            //app.Run(async (context) =>
            //{
            //    await context.Response.WriteAsync("Hello World!");
            //});
        }
    }


public class MyWDVCallbacks : WebDocumentViewerCallbacks
    {
        public override void DocumentInfoRequested(DocumentInfoRequestedEventArgs args)
        {
            Console.WriteLine("**********===DocumentInfoRequested===**********");
            base.DocumentInfoRequested(args);
        }
        public override void ImageRequested(ImageRequestedEventArgs args)
        {
            Console.WriteLine("**********===ImageRequested===**********");
            base.ImageRequested(args);
        }

        public override void FileUpload(FileUploadEventArgs args)
        {
            args.Cancel = true;
            base.FileUpload(args);
        }
        /*
         * The above was just an example of hooking to the two main events.. you can handle any exposed event here
         *
         * //.. most common ...
         *  AnnotationDataRequested
         *  DocumentSave
         *  DocumentStreamWritten
         *  AnnotationStreamWritten
         *  //.. less common .
         *  PageTextRequested
         *  PdfFormRequested
         *  //.. rarely if ever used .. 
         *  ReleaseDocumentStream
         *  ReleasePageStream
         *  ResolveDocumentUri
         *  ResolvePageUri
         */
    }