I create a PDF in Moodle using the following code
$pdf = new pdf;
$pdf->AddPage();
$pdf->Write(1, "Test");
$pdf->Output();
How would I make this download in the browser instead of opening in browser?
Moodle wraps the TCPDF library for PDF generation (the wrapper mostly just handles the locations for temporary files and accessing embedded images which are in the Moodle Files API).
You can find documentation about the TCPDF Output() function online at http://www.tcpdf.org/doc/code/classTCPDF.html#a3d6dcb62298ec9d42e9125ee2f5b23a1
The important param is the second one, calling $pdf->Output('filename.pdf', 'D') will cause it to download.