The most likely reason for this is a corrupt PDF at the top of the file history. To resolve this, follow the steps below:
Make note of the matter reference.
Log in as a supervisor and open Outlook / DPS Oneoffice.
Click User at the top of the screen. If you're using One Office in Outlook then click Addins at the top and then User.
Click Options and then click View.
Change both the preview drop-downs to Off, and then click OK.
Close the window then open the matter.
Double-click the PDF to open it in Adobe.
Print the item, using "Microsoft Print to PDF" and save it somewhere you can easily find e.g. Desktop.
Close Adobe, then right-click the broken entry in the file History and Delete it.
Locate the new PDF, then drag it into the file history to create a working entry.
Open the Addins tab then click User.
Click Options and then click View.
Change both the preview drop-downs back to Bottom or Right.