PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
Doczilla - Effortlessly create stunning PDFs and screenshots. Seamlessly store them in your own AWS or Google Cloud Storage bucket, putting the control and creativity right at your fingertips.
PDFTK Builder - PDFTK Builder is a free graphical interface to for PDFTK, making it much easier to use.
PDFShift - Convert any HTML documents to high-fidelity PDF using a single POST request
PDFChef Merge PDF - PDFChef Merge PDF is an online tool that allows you to merge multiple PDf files into one quickly and securely.
DocRaptor - As the only API powered by the Prince HTML-to-PDF engine, DocRaptor provides the best support for complex PDFs with powerful support for headers, page breaks, page numbers, flexbox, watermarks, accessible PDFs, and much more