As always, the official answer from FileMaker is that older versions of FileMaker have no promise of support or compatibility with OS releases that come out after the end of support.
What PDF issue are you experiencing in 15. A number of solutions have been offered for various issues. One including changing the theme after converting to 15 to make sure you were using a compatible theme. The most issues seem to stem from using the classic theme, which support is being dropped for.