This plugin tries to stay in the background as much as possible to not disturb the daily browsing experience and only shows menu entries when a conversion is available. After you have installed the plugin, it offers you several possibilities when you right click in a webpage:

"Convert page to..." lets you convert the website you are visiting to an image (screenshot functionality), to a document (e.g. PDF), to an E-Book and various other options.
"Convert link to..." converts the data behind the link when you right click on it.
"Convert image to..." converts the image you are pointing at to another format. CSS background images are supported, so you can download and convert them also.

Here is the list of the categories you can convert to:

Archive (e.g. compress a big file before downloading)
Audio (e.g. rip the audio from a video before downloading)
Document (e.g.convert the website to a PDF document)
Ebook (e.g. create an epub of a website to read it later on your ebook reader)
Hash (e.g. create a checksum of a file to verify integrity)
Image (e.g. download CSS background images)
Video (e.g. download a video and convert it for your mobile device)

You can find more information on the webpage: https://www.online-convert.com