Our Exchange Admins installed the IU over the weekend.
This fixed the "Mail.app/non-Exchange-users sending in-line PDFs to Outlook/Exchange users who are not seeing the PDF" issue.
However, there are still issues for Mail.app users -- who are configured for Exchange.
1) If a Mail.app/non-Exchange user sends a message with an in-line PDF to a Mail.app/Exchange user -- the in-line PDF goes to the *bottom* of the body of the message.
2) If a Mail.app/Exchange user sends a message with an in-line PDF to a Mail.app user -- configured for Exchange or something else -- the recipient sees *no* PDF in the body (inline or otherwise). In this case the message in the "Sent" mailbox is also not showing any in-line PDF.
However, in both cases, the attachment is shown in the header of the received message.
So, the IU does not fix all possible (yet common) issues with in-line PDFs.