PDFSaM is free software for splitting and merging pdfs. This is a viable alternative to paid software like Acrobat Pro.
Download the software
Using PDFSaM to merge pdfs
- Go to Plugins>Merge/Extract
- Drag and drop pdfs to the middle area. (or add them via the add button)
- Use tools to reorder/add/remove documents.
- Choose a location and file name to save the document as
- Click Run at the bottom of the window
There are other software and online tools for doing merging pdfs.