I did update everything, as requested, and have done a little more experimenting.  The problem is not quite what I thought it was at first.
  
In File Manager: I have found that I can indeed open any file, however, it can only be done by right-clicking on the desired file and then selecting "open".  It cannot be opened by double-clicking.

In Application Programs:  At first, I thought that these were working OK, but now I discover that to open an existing file, I must highlight the desired file, and then click on the "Open" button.  Again, double-clicking does not open the file.

I have double checked this against my LXLE 16.04.4 machine, and I find that all files can be opened by double-clicking on them, whether in File manager or in the specific application.  

So, at least to me, it seems that my install of 18.04.3 has an issue with opening files by double-clicking.

Thanks.

I researched it on Google.  It is a bug in Ubuntu 18.04.4 LTS.  I wonder if they will bother to fix it.  One Ubuntu user I read said that he fixed it by changing the mouse/touch pad double-click delay timing. (Similar to the solution suggested above), however, there doesn't seem to be any obvious way to do that in LXLE.  Since there is a (less convenient) work-around, a solution isn't mandatory, but it's a shame.  That feature worked perfectly in 16.04.4, but now it's broken.