Just a note for those who face problems on names containing spaces (e.g. "test test.pdf").
In the examples (99% of the time) you can find
header('Content-Disposition: attachment; filename='.basename($file));
but the correct way to set the filename is quoting it (double quote):
header('Content-Disposition: attachment; filename="'.basename($file).'"' );
Some browsers may work without quotation, but for sure not Firefox and as Mozilla explains, the quotation of the filename in the content-disposition is according to the RFC
http://kb.mozillazine.org/Filenames_with_spaces_are_truncated_upon_download