Files that are created and saved in formats generated by specific software applications can in some cases, only be opened/used by those particular applications. In such cases, the usability of the data and other research material is thereby limited.
Original files that are to be archived or shared, need to be supplemented with versions in persistent formats. If you for example want to archive spreadsheets created in Excel, it is advisable to save these as xcl-files with all content (such as graphs or formulas), and as cvs-files combined with a good description (attached in a pdf) of how the graphs or formulas were made. In using this method, you ensure that there is a specific and persistent format available.