Is Windows showing you the full filename including the extension? You can check this by opening File Explorer, selecting the View tab and ensuring that the File name extensions option in the ribbon bar is ticked. Once you're certain this is ticked, navigate to the folder containing your PDF document and check what the filename now reads. You may find that the filename has multiple .pdf extensions at the end - in which case, simply rename the file so that it only has one .pdf extension, then try uploading the document to the portal.
I hope this helps.
Robin