How big are the PDF files? Many internet service providers [ISP’s] limit the maximum size of an e-mail to ’10 Megabytes’. In practice, any attachment larger than 8 or 9 MB will appear to have been sent, but the ISP’s webmail system will not deliver the message with it’s attachment. An e-mail with an attachment always seems to grow in size when sent, probably because of the encryption system which is being used.
My ISP has the 10MB size limit. Yours might be different.
I’m not sure what limit Apple put on e-mails, but I do know that I have sent and received larger attachments through my Apple address than through the one provided by my ISP.
If you are running into a size limit, it might be better to upload the PDF to a secure cloud server of your choice [I use Box, from http://www.box.com, for example], and then share it privately with the intended recipient. A link to the document can be sent to the recipient in an e-mail.