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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.
For my phone (almost 7 years old), a thin needle to scrape out all the dust is sufficient. Pressurized air also helps.