This add-on collects NO DATA at all.

But, theoretically, it can reveal the fact that it's installed (but no other information) to 3rd party websites.
It can happen the following way.

The add-on looks for and process special tags on web pages, e.g.
<embed type="image/vnd.djvu" src="somefile.djvu" />

Such tags are supposed to be rendered by an outdated C++ browser plug-in, which is supported by no modern browsers.
So this addons mimics its functionality and replaces the tags with the DjVu.js Viewer.
These tags can be found mostly on rather old websites.

But if a website from time to time monitors its markup, it can detect that the tags were replaced with the viewer.
But, again, websites will get no other information besides this fact.