It is always possible that a large deleted file has been partially overwritten by other data. As Recover My Files cannot currently preview PST files, the only way to find out is to purchase the software, save the file and test it.
If it has problems, you can use SCANPST.exe or go to
www.recover-my-email.com and run this program to see if the file contains any mail.
If you cant get any mail at all, write to support and explain the situation.