passthru command sometimes fails using PDFTK

I am using PDFTK 2.01 with PHP 7.0.16 on Ubuntu 14.04 to generate .pdf files.

Most of the time, it works fine. Every so often (can be a months apart), I get the below error when pdtfk attempts to execute the passthru command:

Unable to execute '/usr/bin/pdftk my.pdf fill_form tmpname output - flatten''

This causes me to no longer be able to generate .pdfs, unless I restart my server. How do I stop this issue from occurring?

via Chebli Mohamed