Just to verify, DSpace 7 does not have an out-of-the-box PDF viewer at this time. So, similar to other files, it simply sends the PDF file to your browser to let your browser (and computer) decide how to display it.
That said, if anyone finds a PDF viewer that they are able to integrate into DSpace, we always appreciate code/improvements/enhancements donated back to the project. One option might be to look at integrating an Angular PDF viewer like
https://www.npmjs.com/package/ng2-pdfjs-viewer (I don't have any attachment to this particular PDF viewer, and I don't know how hard it'd be to integrate).