It is likely that the email you are forwarding was originally created by 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 If you need to forward an email that had a pdf document, then use : right click on email in list and select: 'Forward as' > 'Attachment'