Have you tried downloading the file before attempting to open it on the Mac?
What app is trying to view the .pdf file when you attempt to open it? After downloading the .pdf file, Right-click or Control-click the .pdf file and select the "Preview" app. If you have other apps installed which can view .pdf files, then you can try selecting one of them to open it as well to compare the results. Sometimes web browsers have their own built-in .pdf viewer (I think Firefox and Chromium base browsers have this feature).
Try booting into Safe Mode and trying to open the file. If this works, then it means you have some third party software installed which is interfering with the normal operation of macOS.