PDFsam Basic 2.2.4 for Windows
PDFsam basic is a free and open source, platform independent software designed to split, merge and rotate pdf files. The installer may include one advertisement offer to finance further development, read here why.
PDFsam Enhanced 2.2.4e
PDFsam Enhanced provides a wider set of features. Get it by donating a minimum amount of 7.99€.
Other platforms or versions
All the available downloads for the supported platforms (MacOS, Linux..), the source code, the MSI installer and the Zip bundle can be found here.
Command Line Interface
If you are planning to split and merge pdf documents from command line you should download the latest version of sejda-console. Sejda is the core pdf manipulation engine for PDFsam v3.
PDFsam version 3.x is the upcoming and currently developed version of PDFsam. It is not yet a stable one but if you are feeling a little brave and willing to test it, you can find it here.