To resolve the "F" or "UF" Key Missing in File Specification error, use the Acrobat Pro DC 2015 Profiles option in the Preflight tool.
The Profile is a selection in the Preflight tool (top, middle of the dialog box).
Once the Profile is selected, select the wrench icon and select the following option from the context sensitive list in the dialog box, "Insert missing F and UF entries for embedded files".
While I do not know what causes this issue, it would seem to be related to typefaces (PDF 32000-1:2008 standard).
Many thanks to Kenny Moore of the 508 Pdf Help Center. I would not have found the solution without his help. Visit his website for many helpful tips on PDF tagging, Tagged PDF | 508 PDF Help Center | Accessibility is the right thing to do.