You can change the source of installation to Play Store by using this adb command: adb shell pm install -i "com.android.vending" -r "path to apk"
You might be interested in https://mupdf.com/wasm/demo/index.html?file=../../docs/mupdf_explored.pdf, as I've seen people recommend this for large PDFs that would lag out even native viewers. I don't know about flickering though as I haven't tried it out much.
What about Cloudflare? You can tunnel to them instead of Tailscale's limited public serve