PDF Combine is a handy application that let you merge several PDF documents into one single PDF file. The merged PDF file will keep the original formatting, page size and page orientation. It also allows you to adjust the PDF sequence by the UP and DOWN buttons, which means you can simply add your PDF documents and reorder the sequence later even if those files are in different folders. PDF Combine has a Mac version named PDF Merger Mac.
Please note: the archive also includes the Mac version of the software