How can I reduce the size of the PDF generated so that I can send it by e-mail?

Presently I'm creating over 400 tax receipts to send out, the majority by post and printing them is going well.  However about 100 of them must be sent by e-mail and because the tax receipt has colorful images the PDG generated is very heavy 12,7 Mo and it's quite long to send by e-mail.  Is there a way that I can save to a lighter file?  Thank you for your help.