Alex went to and installed the real app. Then, in Android Settings → Apps → Firefox → "Open by default," Alex set it to handle the needed links. The PDF opened perfectly — no shady APK needed.
But Alex paused and asked: What is a handler? Firefox Handler Apk
Alex was trying to open a special PDF link from an old website. The link wouldn't open in Chrome. A tech forum said: "Use the Firefox Handler APK." Alex went to and installed the real app