using (var stream = new MemoryStream())
{
    using (var archive = new ZipArchive(stream, ZipArchiveMode.Create, true))
    {
        for (int i = 0; i < 10000; i++)
        {
            Trace.TraceInformation("Starting {0}", i);
 
            var entry = archive.CreateEntry(string.Format("{0}.pdf", i));
 
            var invoice = new InvoiceForm("../../invoice.xml");
            var document = invoice.CreateDocument();
            var renderer = new PdfDocumentRenderer(true) {Document = document};
            renderer.RenderDocument();
           
            using(var rendererStream = new MemoryStream())
            using (var zipEntryStream = entry.Open())
            {
                renderer.Save(rendererStream, false);
               
                rendererStream.CopyTo(zipEntryStream);
            }
        }
    }
    var bytes = stream.ToArray();
}