I am using Affinity Publisher, ver. 1.9.2.1035 and Windows 10. In trying to solve a problem on the Amazon KDP forum, I ran some tests. The problem had to do with a rejected file.  I diagnosed the problem to a font.  This font had Print and Preview embedding rights.  The person was using AffinityPublisher.  Getting some confusing results I: Created a new afpub file Typed the names of 30 Print and Preview fonts, set in their relative typefaces Exported to PDF/X-1a O