Jim,
I just tested it again and it does open. It's not actually a web site link, it's pointing to the file in the file bases here, in the folder "Glenn F Chesnut AA History" file name "History of AA History Lovers Glenn Chesnut.pdf" which is a PDF file that you must download and open with a compatible PDF reader or web browser. All modern operating systems and most devices are compatible with reading this format out of the box. I am not sure what you are using to try to open this link, but I have tested it and it does work on both Windows 10 and Android. Try another device or browser? Download the file first, then try to open it maybe? In Windows, one can "right click" the link and choose "Save Link As" to save the file, for example. And Chrome is an example of a browser that is definitely compatible with PDF files if you do not want to install a standalone reader (this is true on Android, also). On any standard iOS device or Mac, you should also have no trouble opening it. Just tested with an iPad and it worked, as well.
Maybe try a different device to try to isolate the problem? I am thinking it's something going on with the device or software you are using to view the message.
If I can be of any further assistance please let me know.
Best,
Thom
Thom R.