Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
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.
Bootstrap Zero - Open-source, free Bootstrap templates collection.
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
HackerThemes - Bootstrap 4 themes and tools for web developers
Tcpdf - TCPDF: Open Source PHP Class for generating PDF documents