Q: To combine PDF's now, you have to use Print & Save to PDF

I just upgraded to El Capitan and found that this article is now out of date (though it may not have stopped working with El Capitan).

To combine PDFs, drag the thumbnails as described in this article but you'll find that the result is a window with multiple documents in it instead of a single combined document.  To get a single PDF, select Print off the File menu and then choose Save to PDF off the PDF menu at the bottom right.

h/t  http://osxdaily.com/2014/06/27/how-to-join-multiple-pdf-files-into-a-single-pdf- document-in-mac-os-x/