C# converting to jpeg to pdf using iTextSharp dll results in black background color

Question

We have a program that scan first and then store file path in database. While scanning, first the program scans as jpg format and then convert to that file as pdf file. All is ok here. It's has been done successfully.

Only the problem is, for some Epson, HP, Samsung scanners, after scanned and converted to pdf, the file is being converted with black background color and white forecolor of characters which is opposite.

So far, only Canon scanner works as expected.

I have been using the below code for converting process:

 public void JPGtoPDF(string jpgpath)
    {
        iTextSharp.text.Document doc = new iTextSharp.text.Document(iTextSharp.text.PageSize.LETTER, 10, 10, 42, 35);
        try
        {
            //Create Document class object and set its size to letter and give space left, right, Top, Bottom Margin  
            scannedFile = pdfFilePath + "ScannedFile" + Utils.ServerSistemTarihi().ToString().Replace(".", "").Replace(":", "").Replace(" ", "").Replace("/", "") + ".pdf";

            PdfWriter wri = PdfWriter.GetInstance(doc, new FileStream(scannedFile, FileMode.OpenOrCreate));
            doc.Open();//Open Document to write   
            Paragraph paragraph = new Paragraph("");
            // Now image in the pdf file 

            DirectoryInfo dn = new DirectoryInfo(@"C:\Windows\Temp\");
            FileInfo[] fn = dn.GetFiles();
            for (int i = 0; i < fn.Length; i++)
            {
                if (fn[i].Extension.ToLower() == ".jpg")
                {
                    string imageFilePath = fn[i].FullName;
                    byte[] b = File.ReadAllBytes(imageFilePath);
                    iTextSharp.text.Image jpg = iTextSharp.text.Image.GetInstance(b);
                    //Resize image depend upon your need  
                    jpg.ScaleToFit(580f, 560f);
                    //Give space before image  
                    jpg.SpacingBefore = 30f;
                    //Give some space after the image  
                    jpg.SpacingAfter = 1f;
                    jpg.Alignment = Element.ALIGN_CENTER;
                    doc.Add(paragraph); // add paragraph to the document  
                    doc.Add(jpg); //add an image to the created pdf document
                }
            }
        }
        catch (DocumentException docEx)
        {
            //handle pdf document exception if any  
        }
        catch (IOException ioEx)
        {
            // handle IO exception  
        }
        catch (Exception ex)
        {
            // ahndle other exception if occurs  
        }

so what do i have to do to overcome this issue and prevent scanning files depending on scanner brands?

I got checked adobe reader preferences but nothing worked out.

I have also checked scanner original program to setup any features about this matter but I could find nothing useful...

Any clue???