Structurally, the service is hosted online as a website, this means one must upload a copy of their data to be converted. Naturally that raises questions because there's a lot we don't know about this site: who runs it, what are the terms of service (I saw none on the site), what are users granting the site owners to do with uploaded data? Answers to these are almost certainly guesses as this information is not readily available. The site source code is minified Javascript; designed to be terse and at the cost of legibility (variable names are often shortened, useful or long comments eliminated), certainly not the source code a developer would work with when writing the software.
How could this have been implemented in a way that obviates all of these show-stopper failings? Respect users' software freedom by offering users one or more programs to do the conversion jobs with reasonable default settings. Distribute complete corresponding source code alongside the rest of the site so as to let the user host an instance of the site on their own. Users so inclined can set this up for themselves and others, or use their software freedom to end up with a conversion service they don't need to guess about.