Certain formats shrink files without sacrificing the quality of the content. The all-purpose Format Factory free download can also reduce the size of heavy files.
You'll get your files back if you change their format into one your OS can recognize or that you can open with an application installed on your computer. If they do appear, you will see a blank box or a question mark where the file icon should be. Your files are not erased, even if you can't see them on your file explorer. Not all apps or operating systems support or recognize all file extensions.
Some files run well on their original apps or in a Windows Operating System such as 11 and 10 but get stuck or won't open with other applications or in a Mac OS.