Android supports multiple payment providers. Some banks implement their own payment provider (e.g. Sparkasse in Germany), most just rely on Google Pay (now Google Wallet). Google Wallet has strict requirements for the Play Integrity API. Because of the modifications to Android that GrapheneOS is implementing, it is not eligible to receive the required integrity attestation and thus, Google Wallet is refusing to work. Google could at any point reconsider and certify/whitlelist GrapheneOS, which would allow Google Wallet to work using GrapheneOS. Likelihood close to 0.
Any banking app implementing their own payment provider is completely independent of this decision unless it also relies on Play Integrity API attestation (or a similar mechanism).
I am using Librera Reader. Make sure to install the F-Droid version (Librera FD) without Google Play services.
It is primarily an ebook reader that supports a variety of formats, but it is also an excellent PDF viewer. Significantly more feature-rich than any other FOSS PDF viewer for Android that I have found.