I’ve tried probably a dozen PDF readers over the years on Android, and I keep coming back to Moon+ Reader. It’s not exclusively for PDFs, which is part of why I like it—I read a ton of EPUBs too—but its PDF handling is surprisingly robust for a general-purpose app. The scrolling is smooth, the reflow option works okay for text-heavy PDFs when you’re on a small screen, and the customization for brightness and color temperature is way better than most. Where it really wins for me is the folder-based library view. I download a lot of research papers and public domain books as PDFs, and having them organized alongside my other books in one place is a huge time-saver. The offline part is a given; once they’re downloaded, you’re set.
That said, if you’re dealing with complex PDFs like scanned graphic novels or textbooks with intricate layouts, you might want something more specialized. I’ve heard people swear by Xodo for that, and it’s free. Personally, I find its interface a bit cluttered, but the annotation tools are top-notch if you need to highlight or draw on documents. For pure, no-frills offline reading of novels or simple text PDFs, Moon+ does the job without fuss and doesn’t nag you with subscriptions.