* Hongyi Zhao <hongyi.zhao@gmail.com> [2021-07-23 11:57]:
> Other possible processing includes highlighting some regions with
> square or circle,  adding some text comments, and drawing some lines
> or arrows, and so on. I'm not sure whether all these trivial
> operations can be done by the method similar to the above.

If you wish to make a PDF file with annotations or otherwise annotate
other PDF files, I am recommending this application:

Description     : Handwriting notetaking software with PDF annotation support
URL             : https://github.com/xournalpp/xournalpp

Now when you make a screenshot you can then open up Xournalapp and
import the image and make your annotations on the image, save as
PDF. Similar can be done with graphics editors such as GIMP.

See example in attachment.

-- 
Jean

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