If you want to read the book on a mobile device such as a phone or a tablet, you would chose the epub format. That format is used for a variety of software applications, or apps to display the words on the screen based on the screen size and the size of the font you have selected. The words flow according to the size of the screen, referred to as reflowable content. There are no fixed number of pages in that format. You can search for words and make bookmarks to note specific parts.
If you want to read the books on your Kindle, or use the Kindle app on your phone or tablet, you should choose the mobi file. This will work in much the same way as the epub format, and the words will flow on your screen based on the screen and font size. You can also search and make bookmarks.
The last choice is the most common, but it is not designed for use on mobile devices. The pdf format has a fixed image for each page and will always display the same amount of words regardless of the size of the screen. If you are using a laptop or a desktop, that format would be suitable. If you are using a small screen device, you would have difficulty viewing all the text on each page as you would have to enlarge the screen and scroll back and forth. You can search within a pdf, and you have some ability to highlight text.
We now have 13 different Rules of Court in the above formats available here. If you have questions about loading the books, you can ask me.