I’m very new to imposition but I don’t see why you couldn’t use any FOSS for PDFs to rotate and arrange a few files next to each other.
If not doing bulk editing, even a FOSS graphical tool could do the job just fine.
This thread talks about some of those solutions: Are there any free privacy friendly tools to add tiled text watermarks to documents? - #29 by Robin_RM
Otherwise, if you’re comfortable using the CLI: there are even more tools for that matter that a search engine will quickly bring up. 