static PdfBitmap RescaleImage(PdfBitmap src, int newWidth, int newHeight)
{
    PdfBitmap bitmap = new PdfBitmap(newWidth, newHeight, true);
    using (var g = Graphics.FromImage(bitmap.Image))
    {
        g.DrawImage(src.Image, 0, 0, newWidth, newHeight);
    }
    return bitmap;
}

static void Main(string[] args)
{
    PdfCommon.Initialize();
    using (var doc = PdfDocument.Load(@"d:\0\multipage.pdf"))
    {
        foreach (var page in doc.Pages)
        {
            foreach(var img in page.PageObjects)
            {
                if (!(img is PdfImageObject))
                    continue;
                var imageObject = (img as PdfImageObject);
                //Rescale found image to reduce its size
                var newBitmap = RescaleImage(imageObject.Bitmap, imageObject.Bitmap.Width / 2, imageObject.Bitmap.Height / 2);
                //Replace old bitmap with reduced one.
                imageObject.Bitmap = newBitmap;
                //Generate image stream into page resources. This stream will be wrote to page resource dictionary under FXX1 key
                Pdfium.FPDFImageObj_GenerateStream(imageObject.Handle, page.Handle);

                //Replace old image stream in the page resources with newly created stream
                var xObject = page.Dictionary["Resources"].As<PdfTypeDictionary>()["XObject"].As<PdfTypeDictionary>();
                xObject["Im1"] = xObject["FXX1"].Clone();
                xObject.Remove("FXX1");
                imageObject.Dispose();
            }
        }
        doc.Save(@"d:\0\multipage_rescaled.pdf", Patagames.Pdf.Enums.SaveFlags.NoIncremental);
    }
}