Reading and annotating multiple pdf in one window

I am totally disappointed with EndNote. I can see that I have to still use Mendeley for reading my articles. In EndNote, we can’t open multiple Pdf files to read in one tab. This is disgusting. Every time I open a pdf file, it gives me a new window. Now if I open 10 pdf files, there will be 10 windows. Third class feature. Can you improve it or not? Please check Mendeley. How nicely we can read many pdf files at the same time under one desktop window.