After converting an email with a PDF attachment to PDF, the attachment will automatically lock
In order to delete the attachment, it will have to be unlocked first.
This can be done by navigating to the output PDF in foxit PDF Editor after converting the email.
You will see at the header of the converted email, there will be an "Attachments" section with the files that are included on the PDF.
Right click the PDf attachment that you wish to delete from the PDF:
Navigate to Properties
The File Attachment Properties will come up and you can uncheck the "Locked" option to allow for the attachment to be deleted: