How to take selected-area-screenshot to clipboard once it's selected?
Instead of using Samsung select copy paste use the inbuilt options from ezPDF

From the floating 3 dot menu at right centre &rarr; Menus &rarr; Edit &rarr; snapshot &rarr; select the area and take snapshot &rarr; share with &rarr; Squid (choose always since you do that frequently).

The picture gets posted in Squid without switching apps. Here's an example of both apps in split screen mode with transfer of screenshot