So just to play devil's advocate: is it really that much more work to build something entirely new in Tauri than to just do the porting work for releasing a GTK app on Windows/macOS?

I know it's annoying but it's not like it's impossible, right?

It's really easy to build wrapper apps based around good available libraries in tauri.

That doesn’t answer my question at all.

What exactly is so hard about porting the existing GTK app? That work already exists, anything in Tauri is reinventing the wheel.

Calibre is like the mupdf of epub

mupdf is the mupdf of epub; it supports epub and other formats beyond pdf¹. When I've had really large files I've used mupdf to read them a few times, as it seems to be far better at handling them than other tools.

¹ https://mupdf.com/

Is it multi platform though? It seems like the platform is Chromium.

*Webview