This article discusses some of the free methods/tools for merging of PDF files.

PDFsam

For GUI based fans, PDFsam is a good choice with an intuitive interface.

As of now, there are 3 different versions available for download and they are Trial, Pro and Education. The Trial allows for 14 days of free usage with all capabilities enabled. After the 14 day trial, the Merge feature will still continue to function.