Progressive Web Apps vs. Desktop Apps?

Is creating a web application out of it a better apprach?

I'd say yes, but only if you can ensure it will work offline and can work around the general lack of filesystem access.

One of the big things about doing a PWA instead of a desktop app (aside from the triviality of distribution) is that the platform differences you have to deal with become much much less significant most of the time, which makes it easier to support a wide variety of systems (at least, provided you don't worry about Internet Explorer and Opera Mini).

There are a couple of downsides though:

• You need to go out of your way to make sure it works offline if it doesn't actually need to talk to the server (your application sounds like it shouldn't if you do it right, and doing so shouldn't need much effort), otherwise users will tend to avoid it.
• Web apps in general (not just PWA's) have severely limited filesystem access. Saving things pretty much amounts to generating a link (locally, using the Blob API) and telling the user to download it. Loading a file requires jumping through hoops with the [FileReader API]9developer.mozilla.org/en-US/docs/W...). Both have some support limitations across browsers, and are generally a pain to use. Your case sounds simple enough though that this shouldn't matter much (if at all).
• Web apps can't access most hardware directly. Shouldn't be an issue for you unless you really want to do something crazy like offloading the PDF transformations to the GPU (though that may be possible with WebGL).