<Files> directive only applies to filenames, not file-paths, so your
<Files> directive will never match and the header will not be set.
To set this header on a specific file (and not all
.pdf files - as in the linked question/answers) in the root
.htaccess file then you can set an environment variable when this file is requested and conditionally set the header based on this env var.
SetEnvIf Request_URI "/path/to/example.pdf" NOINDEX
Header set X-Robots-Tag "noindex, nofollow" env=NOINDEX
Alternatively, if you could place an additional
.htaccess file in the directory that contains the PDF file you want to target, then you could use a
<Files> directive in that
Header set X-Robots-Tag "noindex, nofollow"
You could use this same method in the root
.htaccess file, but it will also add the header to all
example.pdf file requests on the system - although it's probably unlikely you have more than one file with the same name anyway I would think, so this may be the better solution after all.