Evince - Evince is a document viewer for multiple document formats: PDF, Postscript, djvu, tiff, dvi, XPS...
PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
Ghostscript - Documentation.
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
ReaSoft PDF Printer - ReaSoft PDF Printer is a document creator and virtual printer installer for the Windows operating systems.
PDFill - Open, View, Edit, Save and Print PDF Documents without Adobe Acrobat