It is highly likely that the email with attached .pdf originally came from someone using Outlook. Outlook is known to have issues when attaching pdf so that forwarding becomes a problem. eg: https://community.spiceworks.com/topic/2104231-microsoft-outlook-not-forwarding-attachments the only way to forward emails with attached pdf that originally came from Outlook email client user is to use the option: right click on email in list and selct 'Forward as' >'Attachment' .