Chris  
Alright, it seems nobody has made a good, simple, open source #MusicSheets program for just displaying PDF and adding some annotations to them. Especially not for Linux. There are closed source programs for sale on Windows 10, Android and iOS, but they aren't available for Linux (I haven't tried Wine or Anbox to see if they at least run on Linux).
Jan 19, 2020 at 10:36pmWeb
Show threadKatana SteelJan 20, 2020
@RyuKurisu
Maybe not exactly pdf annotations but definitely a music sheet editor.
http://lilypond.org/easier-editing.html
Show threadChris  Jan 20, 2020

@katana_steel apparently I forgot to mention which apps I've already tried out. What I've currently been using MobileSheets on Windows 10 and MobileSheets Pro on Android (and most features I've mentioned are included in those apps).

Frescobaldi, LilyPond and Musescore all are apps that are great for composing music, but are terrible for just displaying songs that have been digitised (like songs exported from the before mentioned apps or sheets that have been scanned in)