The internet is a gold mine.
However, it is not always possible to have enough spare time to go through pages of materials available online at one go.
How to Save a Webpage as a PDF in Chrome, IE, FireFox, iOS, Android
Searching for the same pages or saving them for later retrieving does not always work.
Compared to others, PDF is preferred as it saves as well as sends document in the exact intended format as the source.
You can make changes in documents as per your requirement (add footnotes/comments, highlight important passages etc.).