This Site Can Convert Files in Your Browser Without Uploading Them
It's a safer way to convert your images, documents, and audio files.

Sometimes you need to quickly convert an image, audio file, or video, so you search for an online tool. The problem: many online conversion tools aren't safe to use, putting you at risk from malware or mining your data.
Vert isn't like that. This is an open source, browser-based tool that can convert the most common image, audio, video, and document formats. It isn't clogged with ads and you don't need to create an account to use it. More importantly: the tool works (almost)entirely in your browser, meaning your files are never actually uploaded anywhere (save for video files, as I'll explain below). I tested this by opening the website, turning off my WiFi, and converting a large batch of images and documents. It worked.
Converting files without uploading them is great from a privacy perspective, but it's also faster—you're not waiting for files to transfer back and forth from your machine to a server.
To get started with Vert, head to the website and add the files you need to covert by dragging them from your file manager or using the upload button. (The homepage lists all of the supported file types, if you're curious—there are more than five dozen of them.) Next, you can choose a format and run the conversion, then click the download button to grab the converted file.

You can alternatively convert all of your files to a single format and grab them all in a single ZIP file. Note that this will only work if all files dropped into the tool are of the same type—that is, video, image, document, or audio files. A mix of images and videos can't all be converted at once, for example, because there's no one format you could convert them to.
It's also worth noting that video files cannot be converted without uploading them to a server, mostly because of how resource intensive doing so in a browser would be. The tool will warn you before uploading anything. According to the website, videos are deleted from the server after you download your file or an hour after you upload them, whichever comes first. It's possible to set up your own server, if this really concerns you.
I think Vert is a very easy to use tool. I tested it with an EPUB file and was able to make both a Word document and a website. I also tested it with various images, audio files, and videos—it all worked well, and quickly. There are a few caveats. PDF files are not supported, for importing or exporting. And some of my older Word documents resulted in error messages, which was odd but not entirely unexpected (the files in question were over 20 years old). Overall, though, this is a very handy tool—one well worth bookmarking.