Solved After 6.3.7 update there are orphaned attachments.

How to reclaim the orphaned attachments? Here's the interesting thing, when I make a post, I embed the PDF so people can read in the post itself. I also include a download link for those  that want to save it directly. 

Interestingly the download link works, but the PDF display is broken. There have been no path changes to the document. 
here's an image of the diagnostics report. 

Can this be rebuild somehow without having to go into mysql and the specific table entry to check what's happening?

If you think that the error has occurred due to the update, you can try to recount the statistics (Kunena -> Tools). But I'm afraid that this won't help.
An orphaned attachment is not displayed because it is not assigned to the post.
This means that the attachment has been uploaded but does not have a post ID. This also explains why a direct link to the attachment works.
You can test how you can obtain orphaned attachments. Orphaned attachments are created when users e.g upload attachments sloppily. This means that the user has used the back button of the browser and the forward button again before saving. Kunena registers this as a cancellation and does not enter a message ID for this attachment.
If you don't want to go into the database, there is only one option. Download this PDF, edit the relevant post and insert this PDF into the post again.

Thank you for the response. I just tried the rebuild the stats option and that did work! The orphans are gone and links re-established. There was still two posts which did not show up as an error in the back end, but didn't have attachments, and like you say, editing the post itself and re-attaching them solved those few.
Thank you very much for this idea, was surprised it was that simple.

Cheers