How to find files that don't contain a given search string

You want to use the "-L" option of 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.

Just add the "L" flag. This does the inverse

find /Users/me/PDFFiles/  -type f -name "*.pdf" -exec grep -HL 'Font' '{}' ';'