Each version can save to the immediately previous version (starting with 6 or 6.1, I think). So 7.1 can save to 7.0. 7.0 can save to 6.1 and 6.1 to 6.0. Thus you need all those intermediate versions. Of course if you used any features in a later version that did not exist in the earlier version, the save will not work.
Lynn