IOrderedEnumerable<KeyValuePair<long, BlobandBytes>> orderedBandB = await DownloadBlobChunks(items).ConfigureAwait(false); //DownloadBlobChunks simply downloads each file from blob storage

byte[] allPDFBytes;
using (MemoryStream ms = new MemoryStream())
{
    var pdf = new PdfDocument();
    var docs = new List<PdfDocument>();
    PdfDocument reader = null;
    foreach (var bandb in orderedBandB)
    {                    
        if (bandb.Value.Bytes != null && bandb.Value.Bytes.Length > 0)
        {
            reader = new PdfDocument(new MemoryStream(bandb.Value.Bytes));
            docs.Add(reader);
        }
    }
    pdf = PdfDocument.Merge(docs.ToArray());
    pdf.Save(ms);
    allPDFBytes = await CopyToPageBlobBuffer(ms.ToArray()).ConfigureAwait(false); //allPDFBytes contains the merged PDF byte array file
}
async Task<byte[]> CopyToPageBlobBuffer(byte[] array)
{
    int size = array.Length + (512 - (array.Length % 512));     //page blobs must be in multiples of 512 bytes or the upload will fail
    byte[] buffer = new byte[size];

    await Task.Run(() => array.CopyTo(buffer, 0));

    return buffer;
}