Telling Internet Explorer How To Open PDF Files
See this article from Adobe: https://helpx.adobe.com/acrobat/using/display-pdf-in-browser.html
It contains the information necessary for you to either set IE to open PDFs within the browser or to force the use of whatever system PDF reader/viewer you prefer. Although the information presumes Adobe Reader the principles apply regardless of your reader of choice.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore