TOPIC:

I found a file called HWriting.java, from the comments I think this is the functionality my customer wanted.
But I do not know how to use it, can you show me some example codes, thanks.

HWriting is now deprecated, You can use Ink class instead (Ink annotation),

For more details, check PDFLayoutView.

I have used Ink class already.
What I want is "variable point size based on velocity", just like this project provided github.com/gcacace/android-signaturepad , do you have plan to implement this feature?

We're evaluating it but not yet planned the development.
The integration between PDF annotations and the project you was linking from GitHub would need a Canvas -> SVG -> PDF integration.
The same should be portable to iOS.

That means it'd take months before we would release a public release.

We will update you as soon as possible.