How to split a PDF document and store it to custom folder in WinRT

        private async void Button_Click(object sender, RoutedEventArgs e)

        {

            Stream docStream = typeof(MainPage).GetTypeInfo().Assembly.GetManifestResourceStream("SplitPDF.Assets.sample.pdf");

            //load the existing document

            PdfLoadedDocument ldoc = new PdfLoadedDocument(docStream);

            List<MemoryStream> ms = new List<MemoryStream>();

            for (int i = 0; i < ldoc.Pages.Count; i++)

            {

                //Create a new Pdf document

                PdfDocument doc = new PdfDocument();

                //import the page

                doc.ImportPage(ldoc, i);

                MemoryStream file = new MemoryStream();

                //save the document

                doc.Save(file);

                ms.Add(file);

                //Close the document

                doc.Close(true);

            }

            Save(ms);

        }

private async void Save(List<MemoryStream> stream)

        {

            //select the folder to store the PDF files.

            FolderPicker picker = new FolderPicker();

            picker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;

            picker.FileTypeFilter.Add(".pdf");

            StorageFolder stFile = await picker.PickSingleFolderAsync();

            //save the PDF document

            for (int i = 0; i < stream.Count;i++)

            {

                StorageFile file = await stFile.CreateFileAsync("sample"+i+".pdf",Windows.Storage.CreationCollisionOption.ReplaceExisting);

                stream[i].Position = 0;

                var buffer = new byte[(int)stream[i].Length];

                await stream[i].ReadAsync(buffer, 0, buffer.Length);

                await Windows.Storage.FileIO.WriteBytesAsync(file, buffer);

            }

       }