There doesn't seem to be much documentation on setting options with this function, but here's the format for setting some basic things:
$optlist = "destination={page=1 type=fixed zoom=1 top=100 left=50}";
PDF_begin_document($pdfdoc, $filename, $optlist );
the '=' character can be replaced with a space, but I think '=' makes the code more readable.
Also, you must include 'type=fixed' to use 'zoom','left', or 'top'. You can set 'page' without it, though.
Hope this helps. And if anybody knows a good reference for the options that can be set here and what the format is, please link.