Pandoc and LaTeX as an alternative to InDesign for book and paper publishing. Obviously not a direct replacement since they are macro languages and not GUI tools, but they are free and open source and capable of creating great looking output if you put in the time. There’s tons of free learning resources out there too.
I would add Sumatra PDF and Okular to this list
And GhostScript!
Pandoc and LaTeX as an alternative to InDesign for book and paper publishing. Obviously not a direct replacement since they are macro languages and not GUI tools, but they are free and open source and capable of creating great looking output if you put in the time. There’s tons of free learning resources out there too.