Microsoft Office applications maintain track of previous four files you have opened. When you go to file menu you will see a list of these last four documents that you can click on for a rapid way to reopen the file.

The number of just utilized files can be altered if you want to Office to show more or less than the default 4 files.

For this go to Tools then Options and then select General tab. There you will see a sector that says Recently used files list. From here you can modify number to anything from 0 to 9.