It isn't the pdf format, it is the likely the file size.
go to Machine Status > Tools > Admin Settings > System Settings > General > Max Email Size and increase it to the max setting.
- What is the attachment size limit on the account on that server? If there is an attachment size limit for the e-mail server, the printer is set to 2,048 KB by default before the e-mail will be sent as separate E-mails. The maximum e-mail attachment size that the printer can be set to is 16,384 KB.