求教参数为Stream类型的文件流和int类型的文件格式，如何在不将文件流保存到本地的情况下，获取到其中的XpsDocument或者Document属性，且无乱码，附上代码片段

private void PreviewFile(Stream fileStr, int fileType)
{
try
{
using (fileStr)
{
switch (fileType)
{
case 1:
moonPdfPanel.Visibility = Visibility.Collapsed;
toolBar.Visibility = Visibility.Collapsed;
documentViewer.Visibility = Visibility.Visible;
byte[] buffer = new byte[fileStr.Length];
fileStr.Read(buffer, 0, buffer.Length);
var doc = new FlowDocument();
MemoryStream memoryStream = new MemoryStream(buffer);
var textRange = new TextRange(doc.ContentStart, doc.ContentEnd);
textRange.Load(memoryStream, DataFormats.Rtf);
MemoryStream ms = new MemoryStream();
Package package = Package.Open(ms, FileMode.Create, FileAccess.ReadWrite);
Uri DocumentUri = new Uri("pack://InMemoryDocument.xps");
PackageStore.RemovePackage(DocumentUri);
PackageStore.AddPackage(DocumentUri, package);
XpsDocument xpsDocument = new XpsDocument(package, CompressionOption.NotCompressed, DocumentUri.AbsoluteUri);
XpsDocumentWriter writer = XpsDocument.CreateXpsDocumentWriter(xpsDocument);
writer.Write(((IDocumentPaginatorSource)doc).DocumentPaginator);
documentViewer.Document = xpsDocument.GetFixedDocumentSequence();
xpsDocument.Close();
break;
case 2:

                        break;
                    case 3:
                       
                        break;
                    case 4:

                        break;
                    case 5:

                        break;
                    default:
                        break;
                }
            }
        }
        catch (Exception ex)
        {
            _isLoaded = false;
            throw;
        }

    }