I was lazy enough to not search for a way to combine .pdf.
This tool is very basic and efficient, just open the terminal:
- drop the tool on the window
- drop your 1st pdf to combine on the window
- drop the 2nd, 3rd, ...
- type (or copy) "-o ~/Desktop/Combine.pdf"
and your combine pdf will be on your desktop.

It's the first time I use it, very simple no problem :-)

PS for the dev:
an automatic output (like ~/Desktop/Combine.pdf) will be more simple, if the user didn't specified it. It will be more ergonomic ;-)
PS2: too bad there isn't a progress bar (but I tried CombinePDFs after your soft, and the progress bar was wrong). It's impossible to have a real progress bar?
PS3: something strange (same with CombinePDFs), the resulting pdf(1,5Mo) is much bigger than the 2 input pdf (912Ko+12Ko)! Seems that the Apple API didn't just concatenate pdf...

bye :-)