I’m not sure about the manuals, but the API reference is compiled from an intermediate format extracted from source code.
I’ve made a Dash docset for the API reference in my spare time based on the intermediate format in case anyone is interested (note: this is not an official Defold project or product). As you can see from the code in the Github project the intermediate format of the API reference is publicly available from our download site. You should be able to convert the same files to a PDF with just a little bit of scripting.