I found that PDshading support has not been added to this repository, and the rendering and creation about shading will also be ignored.
I added support for shading(type 1 to type 7) with the latest version of pdfbox code and the android.graphics library.
Most of the code is just simple modifications of pdfbox.
and unit tests for types 2,3,4 are also added.

The source code is latest two commits in here;

Here is 3 file to test shading 2,3,4:
shading2.pdf
shading3.pdf
shading4.pdf

Please create a new branch named shading to merge and test.
or can i just create a pull request to master?

waiting for you reply,
thank you

Edit 03/23

I fixed an index error bug, and optimized some heavy functions.
big_sm (1).pdf
it would cost about 140s to render the big_sm (1).pdf to images.
Now it would cost just about 33s.
here is the method cost log files:
after_optimize.log
before_optimize.log