WinRt转换PDF图像，提高图像质量

在WinRt应用程序中，我想显示pdf内容。因此，我将所有pdf页面转换为图像。但是质量很差。而且在放大之后，它变得更糟了。

我怎样才能提高质量!?是否可以使用编码器?或者我需要一个更高的宽度/高度值?

            StorageFile pdfFile = await ApplicationData.Current.LocalFolder.GetFileAsync(fileName);
            //Load Pdf File
            pdfDocument = await PdfDocument.LoadFromFileAsync(pdfFile);
            if (pdfDocument != null && pdfDocument.PageCount > 0) {
                //Get Pdf page
                for (int pageIndex = 0; pageIndex < pdfDocument.PageCount; pageIndex++) {
                    var pdfPage = pdfDocument.GetPage((uint)pageIndex);
                    if (pdfPage != null) {
                        // next, generate a bitmap of the page
                        StorageFolder tempFolder = ApplicationData.Current.TemporaryFolder;
                        StorageFile pngFile = await tempFolder.CreateFileAsync(Guid.NewGuid().ToString() + ".png", CreationCollisionOption.ReplaceExisting);
                        if (pngFile != null) {
                            IRandomAccessStream randomStream = await pngFile.OpenAsync(FileAccessMode.ReadWrite);
                            PdfPageRenderOptions pdfPageRenderOptions = new PdfPageRenderOptions();
                            pdfPageRenderOptions.IsIgnoringHighContrast = false;
                            PdfPageDimensions rotation =  pdfPage.Dimensions;
                            Size pdfPageSize = pdfPage.Size;
                            if (pdfPageSize.Height > 1000) {
                                pdfPageRenderOptions.DestinationHeight = (uint)(pdfPageSize.Height * 0.85);
                            }
                            else if (pdfPageSize.Width > 1000) {
                                pdfPageRenderOptions.DestinationWidth = (uint)(pdfPageSize.Width * 1.25);
                            }
                            await pdfPage.RenderToStreamAsync(randomStream, pdfPageRenderOptions);
                            await randomStream.FlushAsync();
                            randomStream.Dispose();
                            pdfPage.Dispose();
                            PdfPagePath = pngFile.Path;
                            PdfPagesPathCollection.Add(pngFile.Path);
                        }
                    }
                }
            }