There seems to be a hidden dependency on the which program.

The way the binary gets discovered is unnecessarily complicated. Instead of starting an external binary (a whole new process), I recommend that the script simply checks the $PATH env variable. The syntax for it is very simple.

Sample solution:

    for directory in os.environ.get('PATH', '').split(os.pathsep):
        potential_path = os.path.join(directory, 'wkhtmltopdf')  # perhaps .exe on windows?
        if os.path.isfile(potential_path) and os.access(potential_path, os.X_OK):
            return potential_path