Multivalent is a old, well-proven, and free tool that I've used for PDF operations many times before over the years. It makes imposition easy, for example.
Last time I tried to find the jar itself and the associated documentation, I found that they were removed or obfuscated from current versions of Multivalent and its website.¹ After some digging, I found vintage copies as I remembered them existing. Those are archived here. As best I can tell, the vintage copies were originally released under the GPL.
You can also browse the Internet Archives' Wayback Machine to see the Multivalent website and tool doc at the time.