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.
DocuPUB - DocuPUB is an online file converter, file merger, file compressor, and file resizing tool.
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.
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
Apache PDFBox - The Apache PDFBox™ library is an open source Java tool for working with PDF documents.