The instructions are in the earlier reply (about #5 in the thread). If that doesn't fix the problem (probably not the cause in your case), what I suggested in the previous response was that a different procedure might work to open PDFs.
From what you described, I can envision three or four procedures that might be used to open the PDF: open the reader and then open the PDF from within it, open the PDF via right-click menu, double click on the PDF in Windows Explorer, or double click on the PDF in a third party file manager program. Another option is to try using a different PDF reader (there are many free ones, just experiment). Still other options are to try viewing the PDF by pointing to the file in the web browser or using a...