Notice the string of characters after the "d=". In a previous post a respondent says that this represents a durable link to the document so it can be found if it is moved. However, Document ID Service feature is not on, and this only happens to pptx files. If you send the link with the d= stuff the recipent cannot open the from the link. If you strip off all the garbage they can open the powerpoint file in the client app right from the link.
As mentioned, this experience is not happening for any other file type. Is there a setting somewhere I am missing to turn that off?