The main MSO to PDF conversion seems to be when a user tries to convert Microsoft Organization Chart as PDF. This cannot be done directly, however you can import organization chart to an MS Office document and save the result as PDF.
There may be another reason why you are looking for MSO conversion. A .mso file can also be found as e-mail attachment when the sender of the e-mail used MS Word or other programs as default text editor for MS Outlook.
Such .mso files cannot be converted to anything, they actually cannot be even opened at all. They are only visible because the receiver of the message does not use Microsoft Outlook and contain special formatting for the actual message. This whole function is used so the recipient sees the document / message exactly the same as the sender. So your best option here is to contact the sender to send you PDF directly.