The general approach I use for such situations is to start poking around in the Developer tools of any browser and finding the Network tab.
Sort by file size to find the largest listing of the PDF file, as this is most likely to be the complete file without missing pages. If you don't see it, try reloading the page. From there, try these in order, from easiest to most annoying.
Method 1: If you're really lucky there will be an accessible URL for the PDF you can download directly. Just right click and open the file in a few tab, then you can save it.
Method 2: Copy the cUrl command and pipe that to a file. In theory this may be possible from any command prompt but I've had the best luck with Linux/WSL.
Method 3: Save as HAR with content, and use something like har-extractor to extract the PDF.