Generated Invoice Filename

I am looking for a way to customise the filename of generated invoices included in emails. I use custom fields as I’m an event based business and would like to include the event name in the invoice name.

I believe you can find the pdf generation function in application/helpers/pdf_helper.php:
Look for:

return pdf_create($html, trans('invoice') . '_' . str_replace(array('\\', '/'), '_', $invoice->invoice_number)