PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
pdflayer - Free, powerful HTML to PDF API supporting both URL and raw HTML conversion. Unlimited document size, lightning-fast and compatible PHP, Python, Ruby, etc.
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
PDFill - Open, View, Edit, Save and Print PDF Documents without Adobe Acrobat
Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents