Wordpress WordPress not recognizing valid PDFtk installation

I need PDFtk package for a Pro2PDF WordPress plugin. I have installed PDFtk package on the centminmod server and the ssh command "which pdftk" outputs as /usr/bin/pdftk
Terminal command pdftk also works.
However, the actual WordPress plugin is unable to recognize the PDFtk install. Normally it would show the path /usr/bin/pdftk, but not in this case.
The plugin support mentioned that pdftk may not be available for all users and shell_exec is not restricted.
I have removed shell_exec restriction from /usr/local/etc/php-fpm.conf
but the plugin is still not recognizing.
How can I make sure that nginx user has access to this pdftk package?

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