It sounds like you may need to re-install some of your fonts.
A box also comes up asking me to select a language that will make the coding readable.
Can you post a screenshot of that box?
Start Adobe Reader and open the file with File > Open...
Looks like the default program associated with the PDFs has been changed to some program other than Reader.
Try repairing the Adobe Reader installation from Control Panel and see if it solves the issue.
Save the file on your desktop (or anywhere for that matter).
Right click on the file and select Open With > Choose Program.
From the list of available programs, choose Adobe Reader, making sure that you also tick the box that says Always use the selected program....
Once you have done this, Adobe Reader should open the document - and future documents you double-click on should also open correctly.