Customizable PDF viewer with self-built PDF.js
At first glances, PDF viewer seems to be as easy as opening a PDF URL in an Iframe; however, not only uncustomizable, but also, if some browsers doesn't support PDF browsing, it could be a problem.
A solution to this would be Mozilla's PDF.js. However, the PDF viewer isn't prebuilt for you in pdf.js-dist, only some minimal JavaScript files.