The project is using mozilla’s pdf.js library for displaying pdfs (this is also used for firefox’s build in pdf reader). As far as I am aware it should be able to work with (allmost) all pdf files.

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.