Key features:
- PDF annotation
- S-pen/stylus support
- Sync (preferably NOT just via Nextcloud…looking at you, Saber)
- Handwriting recognition is a plus, but not required
As mention above, Saber fits the criteria well, but I don’t use Nextcloud and that is the only sync available in that app (unless something has changed in the last couple of weeks). I have a Galaxy phone and tablet that both support S-pen and Samsung Notes is excellent, and free, but they’re not exactly privacy concious, so I’m looking elsewhere. I had considered the Excalidraw plugin for Obsidian, but I’m not sure if that works on the Android version of the app.
My Note 4 died last year so I don’t have any android suggestions, sorry. On Linux I like Rnote, but it’s fairly early in development and I last time I wanted to annotate a PDF I had to do so in Xournal++