Apple mail sending "Untitled attachment #####.htm" to outlook along with other attachments
Do you see these strange attachments in some version of Outlook?

In my own testing it seems to be that if there is anything after an attachment then Outlook interprets it as another unnamed attachment and generates an ATT0001.txt file (I'm assuming different versions of Exchange or Outlook might result in different names for the attachments).

If you make sure that the attached PDF is the last thing in the email then Outlook seems to parse the emails okay.

About the Insert attachments at End of Message option on the Edit -> Attachments menu in Mail. By default if you drag an attachment onto a Mail message window it'll place the attachment wherever you drop the attachment. But if you select the Insert attachments at End of Message option, then when you try to attach a file Mail will automatically drop it at the end of the message no matter where you actually position the cursor as you drag the attachment over the window. However it won't prevent you subsequently typing text after the attachment, which results in the ATTxxxx.txt file.

Long story short, make sure the PDF attachment is at the very end of the email and Outlook should parse the email okay.

Let us know how you get on.