Sorry for being late here.
If you are having Adobe Acrobat (full version) installed in your system, you can merge any number of .pdf files into one. I'm using Adobe Acrobat (XI) and do such thing by using these steps:
Or if you do not have Adobe Acrobat full verison, you are not be able to perform the process of combining .pdf files by using these steps. In such situation you need to go a third-party software.
I suggest you to Google.