Thanks for caring about file size!
The reason I never had to install Adobe Reader for more than a decade now is that Sumatra was/is such a tiny install. Thanks!
Edit: I just checked and Acrobat Reader requires 450/900/380 MB for Win32/Win64/Mac respectively [1]. One might argue that it does more than just read PDFs. But in many cases, reading PDFs is all that I need.
What about Tk?
If you are counting MBs at that order, then wxWidgets would be the best option. It can be statically linked easily. The size overhead is about 2.5-3 MB. It is a thin wrapper on top of native controls, so e.g. you can always get to the underlying HWND on Windows. Even for the Windows only app, I'd still pick it due to the sane API.
That's fair in the sense that I did not look closely at latest Gtk or Qt or WxWidgets.
That being said they certainly did not get lighter.
Last I checked Qt was over 10 MB of libraries). Sumatra is 12 MB and I'm guessing over 8 MB is fonts needed to render PDF documents.
So just Gtk or Qt code would be more than the whole app.
Latest Gtk4 does seem to look nice so maybe calling it ugly was uncalled for.