'PDF Header Signature Not Found' at the time of merging multiple pdf file

Hello All,

I am getting error 'PDF Header Signature Not Found' when i merge to multiple PDF File.

code is below

public string MergePdf(List<byte[]> sourceData)
    {
        string sMergedPDF;
        sMergedPDF = Guid.NewGuid().ToString().Replace("-", "_") + ".pdf";
        string sMergedPdfWithFolderName = System.Web.HttpContext.Current.Server.MapPath("~/TemplateTemporaryFile/") + sMergedPDF;
        try
        {
            int f = 0;
            PdfReader reader = new PdfReader(sourceData[f]);
            int n = reader.NumberOfPages;
            Document document = new Document(reader.GetPageSizeWithRotation(1));
            // step 2: we create a writer that listens to the document
            PdfWriter writer = PdfWriter.GetInstance(document, new FileStream(sMergedPdfWithFolderName, FileMode.Create));
            // step 3: we open the document
            document.Open();
            PdfContentByte cb = writer.DirectContent;
            PdfImportedPage page;
            int rotation;
            // step 4: we add content
            while (f < sourceData.Count)
            {
                int i = 0;
                while (i < n)
                {
                    i++;
                    document.SetPageSize(reader.GetPageSizeWithRotation(i));
                    document.NewPage();


                    page = writer.GetImportedPage(reader, i);
                    rotation = reader.GetPageRotation(i);
                    if (rotation == 90 || rotation == 270)
                    {
                        cb.AddTemplate(page, 0, -1f, 1f, 0, 0, reader.GetPageSizeWithRotation(i).Height);
                    }
                    else
                    {
                        cb.AddTemplate(page, 0.97f, 0, 0, 1f, 0, 0);
                    }
                }
                f++;
                if (f < sourceData.Count)
                {
                    reader = new PdfReader(sourceData[f]); -------getting error
                    // we retrieve the total number of pages
                    n = reader.NumberOfPages;
                }
            }

            // step 5: we close the document
            document.Close();

        }
        catch (Exception ex)
        {
            ExceptionHandler.LogException(ex);
        }
        return sMergedPDF;
    }

if any one have any idea let me know please.

Thanks ,

Khushboo Nayak.