\StringUtil::parseSimpleTokens and Isotope Notificatoin center ##document##

Can some one please explain how the simple token ##document## is converted to a file path?

Inside \StringUtil parseSimpleTokens() function there is a block of code, what and how does it do the conversion of ##document## to a file path?

PHP Code:

---

        // Eval the code
        ob_start();
        $blnEval = eval("?>" . $strReturn);
        $strReturn = ob_get_contents();
        ob_end_clean(); 


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I am using module notification-center to send email from the submitted forms. With the mail, I am attaching a dynamically generated PDF file containing some information from the submitted from POST. I am able to register a custom simple token ##myPDF## and generate the PDF using prepareFormData HOOK. But struggling to resolve the simpleToken ##myPDF## conversion into the file path.