@MediaActivist Here are some of my favourites :)
- Syncthing for sharing files among devices (without need for a server, because it's peer-to-peer)
https://f-droid.org/en/packages/com.nutomic.syncthingandroid/
- Markor for Markdown notes (great in combination with Syncthing e.g. for diaries)
https://f-droid.org/en/packages/net.gsantner.markor/
- VLC -- for playing/streaming any kind of media
https://f-droid.org/en/packages/org.videolan.vlc/
- KOReader -- best ebook reader
https://f-droid.org/en/packages/org.koreader.launcher.fdroid/
- Imagepipe -- crop and remove meta data of images before sharing
https://f-droid.org/en/packages/de.kaffeemitkoffein.imagepipe/
What's still missing: A foss mobile äpp for viewing PDF and making PDF annotations.