How do I make this function thread safe?

[Dr.No]

My application create some HTML which is then converted to PDF by 
wkhtmltopdf library. I'm trying to figure out how make the PDF 
generation run parallel, currently, it's running linearly. My 
guess is wkhtmltopdf internal variables is preventing 
parallelization. But I'm new to parallization and I don't know 
how to solve that by now. I guesses that that function from an 
external thread would make each wkhtmltopdf initilization run on 
its memory space, similiar to a process. But since it is not 
working, I guess this isn't how it's working.
I'm not asking to just give me the code ready (if it's somehow 
complex) brather some directions, how I may archive that.


Here's my current code:

void genPDFImplt(string htmlFilename, string outputpdfFilename)
{
	import pdf;
	import std.string : toStringz;

	/* Init wkhtmltopdf in graphics less mode */
	wkhtmltopdf_init(0);
	wkhtmltopdf_global_settings *gs = 
wkhtmltopdf_create_global_settings();
	/* We want the result to be storred in the file called test.pdf 
*/
	wkhtmltopdf_set_global_setting(gs, "out", 
outputpdfFilename.toStringz);
	wkhtmltopdf_object_settings *os = 
wkhtmltopdf_create_object_settings();
	/* We want to convert to convert the qstring documentation page 
*/
	wkhtmltopdf_set_object_setting(os, "page", 
htmlFilename.toStringz);
	/* Create the actual converter object used to convert the pages 
*/
	wkhtmltopdf_converter * c = wkhtmltopdf_create_converter(gs);
	static if(0) {
		/* Call the progress_changed function when progress changes */
		wkhtmltopdf_set_progress_changed_callback(c, 
&pdf_progress_changed);
		/* Call the phase _changed function when the phase changes */
		wkhtmltopdf_set_phase_changed_callback(c, &pdf_phase_changed);
		/* Call the error function when an error occures */
		wkhtmltopdf_set_error_callback(c, &pdf_error);
		/* Call the waring function when a warning is issued */
		wkhtmltopdf_set_warning_callback(c, &pdf_warning);
	}
	scope(exit) {
		/* Destroy the converter object since we are done with it */
		wkhtmltopdf_destroy_converter(c);
		/* We will no longer be needing wkhtmltopdf funcionality */
		wkhtmltopdf_deinit();
	}
	wkhtmltopdf_add_object(c, os, null);
	/* Perform the actual convertion */
	wkhtmltopdf_convert(c);
}


called from a loop like this:

foreach(string file; parallel(files)) {
    auto res = doSomething(file);
    auto htmlfile = genHtmlFile(res);
    auto pdffile = genTmpPDFFilename();
    genPDFImplt(htmlfiel, pdffile);
}



Running in a dual core CPU, with 4 threads. It's genrating
one PDF per iteration rather 4. I've made sure already it's the 
genPDFImplt() which is doing that.