Apparantly sharepoint online SEARCH does not recognize an .MPP document really as a document.
When I search for an MPP file it will find it but it but when clicked for opening it opens the property form for that document in stead of the document itself.
in the search result it is showed as
while for other types of documents it is showed as for example
However, when I navigate to the library where the document actually is and open it , it opens nicely with MS project.
That's why i think there is a search issue for MPP files in sharepoint online.
Thanks for the replies.