Tooling for Cross-Platform Handbook?

I am starting a medical residency, and many departments at the hospital I am at have paper/PDF handbooks with in-house protocols and resources. They are often a little out of date and hard to find/use on your phone.

I'd like to convert these to a very simple app to improve searching, navigating, and updating the content.

Background: I am an ex-software developer, but mostly backend/scientific-computing and have never built an app so I'm unfamiliar with the territory.

What framework do you suggest for creating an iOS/Android app of what is essentially a reference book with search (i.e. table of contents, chapters, sections with text and images)? My development environment is linux, currently. I'm looking into frameworks such as flutter and react native.

I think my ideally the handbook content would be stored in a public repository, perhaps as markdown, so that it can easily accept contributions / issues. Separate code bases would turn the content into an app, PDF document, or website possibly.

Thanks!