Looking for app installed on Website and allows visitor to select downloadable format of file

Greetings from the other end of the planet!

I DID have this program prior to having a stroke AND collapsed Hard Drive at same time. (Cause?)

As my market is older folk, everything needs to be as easy as possible and this little program could convert our standard web files into E-Reader format for many devices. Visitor simply selected it and nominated the format and hit the download button.

As a newby to this side - it would be a good idea to have a translator service on my website to convert the e-books to an end-user choice and download, or send them here at once?

On research to date, virtually none of them were familiar/happy with the idea of not simply doing it online.

THIS is a very comprehensive Site here! Congratulations and thanks for it.