As software is updated so is the file format associated with the software. Overtime, older formats are slowly phased out to the point that newer versions of the software cannot open the old files. In other cases, software is completely discontinued and their associated file formats become obsolete. If you have files saved in old or obsolete formats you might as well not have saved the files.
In an effort to prevent, you can periodically re-save your files in the newest version of the software format. When you are no longer actively using the file save it in a preservation format. These preservation formats are chosen for their likelihood of surviving through software updates and company closures.
For each type of file the recommended file formats are listed in order of best to worst.
Text (Produced in a text editor or word processing application)
Vector-Based Images (Diagrams, Graphs and Drawings created via software applications)
In an effort to retrieve older files that have become obsolete and prevent the problems associated with proprietary file formats open source versions of common software was created. OpenOffice and LibreOffice are two community developed office software suites that are free to download. These suites can open older versions of commonly used software applications like WordPerfect and allow you to save these files in updated formats like Portable Document Format (PDF) and Open Document Format (ODF).