Website to app

You should consider PWA first, that will be the cheapest solution to convert a website into an offline app

If you list what you need in detail is easier to define the right tool. But if the only need is "offline" the PWAs cover the problem pretty well

Android have amazing support for them. And iOS 26 added good offline features

Short answer: yes, Flutter can do this. I've done similar work in Flutter.

In the end, two things mattered: how you store the data, and how you show it on screen. Offline, you can save it in whatever structure you want, JSON or SQLite.

So store it the same way everywhere, and put your effort into the engine that converts the stored data into the screen. That's the same in Flutter and on the web. Whatever language you use, that converting engine is the most important part. In my case the app and the web use the same note format, and each side just draws it its own way.

For sync, I kept it simple. Before saving to the server, the app checks the server's last-modified time. If the note was changed on the web in the meantime, it doesn't overwrite it. It keeps the phone's version as a local draft and tells the user. Same when a save fails because there's no connection: the edit stays as a draft, and when you open the note again you can restore it and save. Nothing gets dropped silently.

You could maybe use a wrapper like CapacitorJS. I would have to look at your code as further to help.

Do you mean editing the downloaded files themselves (PDFs, spreadsheets, etc.), or changing form/database records? I'd pin down one real document type and the exact edits that must work offline before choosing RN, Flutter or a wrapper. Viewing a cached document isn't the same acceptance check as editing it. Also, does “modify all from one location” mean one UI codebase or one set of business rules? Those are two separate reuse goals.

I have a whole emulator wrapped in capacitor. I am currently changing technology because I was pushing the limit. But something like capacitor is most definitely good enough for most use cases.

It is however always better to have a native experience I think. If it’s worth the rewrite you can use RN as base for your native and web app. Expect to do some specifics for web to make it feel nice though because RN is not very optimized for web.

the offline edit sync is the hard part not the UI. Pick React Native and start with MMKV for storage plus SQLite for your records right now. Capacitor breaks here because webview local storage gets wiped too easily on iOS during system cleanup.

if you are interested - I am testing my solution to move from web mobile to native mobile [swift/kotlin] and searching for 3 people - here the post, https://www.reddit.com/r/Base44/comments/1xdnwvd/comment/19rnoze6hv0rl/
if this is someting for you, I am happy to include you.