Found out part of what is happening. The PDF files are actually loading, but unlike previously it is only showing the first page. At the bottom there is an "arrow" [actually a triangle] that turns the page to read the rest of the PDF file (one page at a time.) There no longer seems to be a way to open the PDF in Adobe Acrobat Reader. When you double click on the file, it opens in Reader for a few seconds, then closes, and Avast anti-virus says it is protecting you from a virus.
PDF files that I have previously loaded now act the same way. You CAN chose "Save As" to place the PDF back on your desktop, and then open it in Reader.
