Replace ng-pdfviewer with viewerjs.

Both ng-pdfviewer and viewerjs uses pdf.js to render PDFs. Viewerjs
though is far more functional out of the box, as you can highlight text,
zoom in/out, etc.

The benefit of ng-pdfviewer was that it's already integrated with
Angular. Unfortunately, its function set is just too limited, it doesn't
have zoom in/out, text highlighting, etc. Most importantly, it can't
show 2 PDFs side by side.

One common complaint from the user study was that comparing answer PDFs
was difficult, since it's in a new window. With viewerjs, we can show 2
PDFs side by side. Some work still needs to be done to integrate
viewerjs with Angular though.