If you're looking for a simple way to merge and split PDFs, pdfAssistant is a great option that's useful for a lot of other PDF chores, too. You can use its conversational AI interface to merge and split documents, among other things. You can sign up for a free account to try it out, and it charges with a credit system that doesn't expire, so you can pay by the job if you're using it for personal or work tasks.
Another good option is LightPDF, an AI-powered tool that can handle a lot of PDF chores, including merging and splitting. LightPDF also can convert PDFs to and from other formats, edit PDFs, annotate them and optimize them. It's available as a desktop tool, mobile app and web interface, and is good for personal or professional use.
PDFgear is another good option, especially if you're looking for something that's free. It's got a lot of PDF abilities on Windows, Mac and mobile devices, including the ability to merge and split PDFs. PDFgear also can perform OCR, convert to other formats, and has an AI-powered editor for interactive PDF editing.
If you want a single tool that can handle a lot of PDF chores, HiPDF is a good option. It's got a collection of PDF tools, including merging and splitting PDFs, and works in multiple languages. It's got 10GB of free cloud storage and international security certifications, too, so it's good for individuals and businesses.