UPDATE: If you installed this script today (2016-08-15), you may need to update it. I added a line in the script that should prevent duplicate pages from being appended.
There are plenty of apps that help you merge PDFs into a single file, but if you want something faster with a “native” feel, you can set up an OS X service to quickly merge selected PDFs simply by right-clicking them.
Setting It Up
First, open Terminal an enter this command to install the Python module that makes this possible:
sudo easy_install PyPDF2
Open Automator and choose Service
Select files or folders in Finder.app using the pulldown menus near the top of the window that opens.
Search for get finder, then drag Get Selected Finder Items from the left pane into the right.
Next, search for script, drag Run Shell Script over. Using the pulldown menus, choose /usr/bin/python for the Shell: and as arguments for Pass input:
Paste in the script below.
Your final product should look like this:
Finally, save the workflow as Merge PDFs and it will become available in your right-click menu when using the Finder.
I have found this to be especially useful for when I need to submit a form with an attached document such as when submitting medical reimbursements or other medical requests.