Using iTextSharp to compress ALL images in a PDF file with a Windows Store App

Question

I used tags in the title, but I think in this case it is needed ;)

I am trying to compress all Images of a pdf file with a Windows store app using iTextSharp.

Through Google I didn't find any solution to this, but I found different approaches that I tried to Combine.

The code runs without an error, but it doesnt seem to do anything.

public async Task startCompressing()
    {
        var stream = await input.OpenAsync(FileAccessMode.ReadWrite);
        PdfReader reader = new PdfReader(stream.AsStream());

        Document doc = new Document();
        doc.Open();

        //Create our output PDF
        using (MemoryStream ms = new MemoryStream())
        {
            //Bind a stamper to the file and our reader
            using (PdfStamper stamper = new PdfStamper(reader, ms))
            {

                for (int i = 1; i < reader.NumberOfPages; i++)
                {
                    PdfDictionary page = reader.GetPageN(i);
                    //Get the xobject structure
                    PdfDictionary resources = (PdfDictionary)PdfReader.GetPdfObject(page.Get(PdfName.RESOURCES));
                    PdfDictionary xobject = (PdfDictionary)PdfReader.GetPdfObject(resources.Get(PdfName.XOBJECT));
                    if (xobject != null)
                    {
                        PdfObject obj;
                        //Loop through each key
                        foreach (PdfName name in xobject.Keys)
                        {
                            obj = xobject.Get(name);
                            if (obj.IsIndirect())
                            {
                                //Get the current key as a PDF object
                                PdfDictionary imgObject = (PdfDictionary)PdfReader.GetPdfObject(obj);
                                //See if its an image
                                if (imgObject.Get(PdfName.SUBTYPE).Equals(PdfName.IMAGE))
                                {
                                    //NOTE: There's a bunch of different types of filters, I'm only handing the simplest one here which is basically raw JPG, you'll have to research others
                                    if (imgObject.Get(PdfName.FILTER).Equals(PdfName.DCTDECODE))
                                    {
                                        //Get the raw bytes of the current image
                                        byte[] oldBytes = PdfReader.GetStreamBytesRaw((PRStream)imgObject);

                                        //convert ByteStream to InMemoryRandomAccessStream to be able to create a Bitmap
                                        var newStream = new InMemoryRandomAccessStream();
                                        await (newStream.AsStreamForWrite()).WriteAsync(oldBytes, 0, oldBytes.Length);
                                        await newStream.FlushAsync();

                                        var decoder = await BitmapDecoder.CreateAsync(newStream);
                                        var memStream = new InMemoryRandomAccessStream();
                                        var encoder = await BitmapEncoder.CreateForTranscodingAsync(memStream, decoder);

                                        encoder.BitmapTransform.InterpolationMode = BitmapInterpolationMode.Fant;

                                        try
                                        {
                                            await encoder.FlushAsync();
                                        }
                                        catch (Exception err)
                                        {
                                            throw err;
                                        }

                                        //Create a new iTextSharp image from our bytes
                                        iTextSharp.text.Image compressedImage = iTextSharp.text.Image.GetInstance(memStream.AsStream());
                                        //Kill off the old image
                                        PdfReader.KillIndirect(obj);
                                        //Add our image in its place
                                        //stamper.Writer.AddDirectImageSimple(compressedImage, (PRIndirectReference)obj);
                                    }
                                }
                            }
                        }
                    }
                }
            }
        }
    }

If I comment out the line:

stamper.Writer.AddDirectImageSimple(compressedImage, (PRIndirectReference)obj);

in my understanding I should lose the Images on all pages. Unfortunately this doesn't happen. It seems like nothing happens. Ignoring whatever I do to the streams and if thats correct,, at least this line should be enough to kill the images.

PdfReader.KillIndirect(obj);

As this doesn't happen, it seems like I have some Problems in my code, that I cannot figure out. The code runs without an error, executing all the lines (The check for jpeg is true aswell)

I couldn't find anything regarding compression with iTextSharp in Windows Store Apps. Maybe you can help me out?

Thanks for your help!