I finally got Office for Mac 2008 installed in my MacBook. Many would say that I could have just gone for an OpenSource program like Neo Office or Bean (both of which I tried and liked immensely except that they don't seem to have the auto-capitalization working), but since I could get Office at a very decent price, I decided to go for it.
Like the version for Windows, the new Office automatically defaults the format to the new "x" file type: .docx, .xlsx, and .pptx, if I'm not mistaken. I encountered this problem in Windows a few weeks back. My mom sent me an Excel file that I had to print but I couldn't open it. A quick Google search helped me identify it and an online converter helped me turn it to a document my older Excel could read.
Eventually, I taught her how to pick the extension .doc everytime she saves it.
My uncle, a recent Mac user, also had problems with the file extension and manually had to change it everytime he saves a document. Sometimes he forgets, so when he sends the document to his clients, they can't open it. Not everyone has made the switch to the new Office in either platforms
Tinkering with Word the other day, I found out that you can change the default file type your document will save in. I think it will also work for Excel and Powerpoint, but I haven't really tried.
On Word, choose Preferences
Under "Output and Sharing", click "Save"
Locate the option that says "Save Word files as" and choose "Word Document 97-2004 (.doc)"
Click on OK.
The next time you save a new document, it will automatically pick .doc. You can change it to whatever format you want though, and this method is only for those who need to send files to others who use an older Office.
Although I hear Neo Office can open .docx files. :)