I store it in paperless. I save it there for the longterm but also to have it ready when needed. If I guess I don't have internet. I save it on my phone as well.
I didn't realize this was something I'm missing too. An alternative to Google wallet would be nice. Right now I usually either just get them from my email client or maybe sync them with nextcloud or obsidian to my phone.
Take a look at Green Pass. There's no selfhosted backend, but the android app makes all your PDFs easily accessible, without fumbling through a files app.
On top of that, I use syncthing to sync the paperless files to my mobile phone. You can set storage path for each file in paperless; I have one for important files like tickets. Then I have them even without internet access 🙂👍
I also use email and company apps (flights, ..) .. 🫣
For movies I also use "note to self" in signal to quickly store and find them..
Save them as PDFs and store them in your cloud storage or choice or a syncing tool like Syncthing. A basic folder structure can help keep things organised.
No need for anything complicated, for essential documents best to keep its impel and limit the scope of failure.
I use Syncthing to sync the documents between my home server and all other devices. Additionally I use Obsidian as my interface (for this many other functions) but you could just access the files directly with a file explorer.