PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
IronPDF - SDK for creating and editing PDFs.
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
PDF to X - PDF to X is used to convert PDF documents to Word (.doc)/WordPad (.rtf) documents, HTML web pages, JPG/PNG/BMP/TIF/GIF/PCX/TGA images and plain text files in batch mode. It works without Adobe Acrobat or Adobe Reader.
PDFill - Open, View, Edit, Save and Print PDF Documents without Adobe Acrobat
Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.