A service Merge PDF.net is what we are talking about here. Yes, MergePDF.net will let you take different PDF documents together and combine them in the order of input to create a single PDF file. Cool, aint it? The best part is you are not even charged a single penny for all this and is entirely web-based, you are not required to download or install any client software.
As with most free things in this world, they come with a limitation, the same holds true for MergePDF too. There are limitations to the maximum number of input files (10), and maximum individual file size limit is 5 MB. Other than that, you are free to merge as many PDFs as you like. I wish the guys at MergePDF launch a paid version of the service where all restrictions are lifted off or may be loosen their restrictions a bit for time-being.