Just what you want to merge PDFs

I just spent an hour dragging and dropping 160 files one by one into the sidebar of Preview and when I exported to PDF ended uo with just the first file in the resulting PDF. I didn’t use the Save command because the an article on the Web cautioned not to use it because it was buggy in Yosemite. Then I inadvertently closed the laboriously created PDF. Face palm! No, I am not going to do that again unless I have to! So I checked the APP store and found PDF Merger Free, installed it, and dragged all the files at once to the drop ‘em here pane in PDF Merger Free. It took a while - the files ranged from 250-300 KBeach , but after a couple of minutes there was my merged PDF. With all the pages, in the right order - first time. Zero problems.

Lgude about PDF Splitter + PDF Merge