What is the best way to Compress PDF file size

Question

I am using itextsharp for generating PDFs.We generated PDFS for the purpose of Brodband bills or cable invoices for individual customers in large amount.Here we want to compress the PDF file size.I tried to compress PDFs through programming like writer.SetFullCompression() and writer.CompressionLevel = PdfStream.BEST_COMPRESSION. But its compressed the file size 55kb to 54kb its not a satisfied result. So there is any better way to compress PDFs. And what is best tool for optimizing PDF file size.