Dave, I have updated my profile, and it should now show 10.6.6. And you will have to forgive an old man, but when you ask about the clock speed, I don't know what that means.
Thanks, it's not a big deal normally. If you ever need to know, though (in certain circumstances it can help to determine the specific iMac model) you can run System Profiler and hook at the Hardware Overview and it will give you that information.
I'm using Safari version 5.0.3. You are right, it's the browser window which goes black, not the entire screen. The menu bar is there. Otherwise Safari is working normally.
Clearly Safari is trying to show the PDFs and failing. Do you recall installing any Safari Extensions, or anything to do with Adobe Acrobat? Either might be blocking display of a PDF. If not, just clearing the cache in Safari (Empty Cache command under the Safari menu) might help.
For a workaround, if you control-click on a link to any PDF, you should be able to choose Download Linked File and download the PDF, after which you should be able to open it with Preview or Adobe's Reader or Acrobat (the first command will save the file to whatever folder you have set in your preferences for downloads, the "Download ... As" will allow you to select where the download goes). You could also try a different web browser such as Firefox and see if that does anything different.