Vuoi usare l'opzione "-L" di grep
:
-L, --files-without-match
Only the names of files not containing selected lines are written to standard output. Path-
names are listed once per file searched. If the standard input is searched, the string
``(standard input)'' is written.
Basta aggiungere il flag "L". Questo fa l'inverso
find /Users/me/PDFFiles/ -type f -name "*.pdf" -exec grep -HL 'Font' '{}' ';'