It is not planned right now as I never developed an android app before. But who knows, I am also not a frontend dev, yet here we are :D

I am planning on developing a Rest API, hopefully I can do this in such a way that someone from community is able to create a mobile application.

No this is not possible. However I think, that you can adjust the page size/scale with Stirling PDF.

It depends on you use case. Stirling PDF focuses on performing various operations like splitting, cropping and rotating on your PDFs. PdfDing has a different focus, it is all about reading and organizing your PDFs. I started this project because I wanted a web app where I can read longer PDFs seamlessly on my desktop and mobile devices.

The newly added editing features were implemented with the aim of improving the reading experience. If I find something important I can add an annotation or highlight something. When studying you can add free hand notes to your files.

I hope that helps in differentiating the two applications.