How and from what are you creating the PDFs, including what joboptions? For what are the PDFs intended?
You could break down trhe PDFs into smaller ones (fewer pages) or you could Zip the PDFs before sending by email.
You could print your PDF to the Acrobat Distiller to create a new PDF (tho all links/forms etc would be lost)
You have asked a very open-ended question... some more details would be useful.