OS X has a built in application that lets you open and read pdf files. It is called Preview. If you click on a pdf file it will automatically open in Preview.
Thankfully with OS X Yosemite (OSX 10.10) it’s really easily merge 2 pdf files with the built in OS X Preview app.
From within preview you can add more pages to the pdf file. When you save the pdf file these extra pages are combined so that all the extra pages are merged with the original pdf.