Multi-tab pdf reader and a way of opening pdfs in the same tab but not from within the app?

Yes there is a way:

for qpdfview:

• copy /usr/share/applications/qpdfview.desktop to ~/.local/share/applications/qpdfview.desktop
• edit ~/.local/share/applications/qpdfview.desktop so that the Exec line reads Exec=qpdfview --unique %f

for okular there might be a similar way yet i did not find one

For Okular you have to check this : Settings->Configure Okular->General->Program Features->Open new files in tabs.

Mendeley, primarily a reference manager, has a built-in pdf reader. PDF files open up in Tabs from within or outside the application. Just try it and it may well become your default PDF reader app due to speed and convenience.