JDoc4droid allows to download and view Javadoc™s.
It is possible to download official Javadocs of Java™ versions like Java 8, Java 11 and Java 17 as as well as JavaFX 8.
Aside from this, JDoc4droid comes with a feature allowing to use Javadocs from Maven Central or other Maven repository if the libraries ship with a Javadoc JAR.
Finally, it is possible to use local Javadoc JAR/ZIP files with JDoc4droid.
JDoc4droid is free and open source software. The source code is available at https://github.com/danthe1st/JDoc4droid.