Oooh, I hope you have fun!! Sorry for the slow reply, I've been working on a huge update. I hope it doesn't disrupt your game too much! It's going to have propper NPC statblocks, and loads of reworks to your abilities! You may have to respec your characters a little bit.
As for your question- I used Scribus! I chose that program because I didn't want to endorse adobe, and I tried to look for a more indie program that can handle buttons and checkboxes. Buuuut Scribus is a little dated and it's challenging to work with so I don't know if I'd recommend it.
I kinda wish that Affinity Publisher would add support for buttons and check boxes! That's the PDF editor that I use for the main book.