DocuPUB - DocuPUB is an online file converter, file merger, file compressor, and file resizing tool.
Documents by Readdle - A central hub for all your files.
PDF Joiner - This online tool allows you to join several PDF documents into a single file, free of charge.
FileApp - FileApp is an iPhone and iPad File System and Document Viewer.
Ultra PDF Merger - Ultra PDF Merger is a freeware PDF utility that joins your PDF files, or part of them, together.
FE File Explorer - The one-stop files solution