PDF files can be nice, because they are made for printing and preserve all layout, just as the creator intended. If you wanted to print the document on your printer, you are welcome to, but just be warned the book are several hundreds of pages long, and I believe in saving the trees.
The downfall to PDF files is they are built for printing and don’t scale very well on mobile devices such as phones and tablets. Also because they aren’t really an ereader format, they don’t remember where you are in your reading.
Use an eReader
I highly recommend downloading the epub files as those can be utilized by the ereading application of your choice.
The advantages to using an ereader application is the text scales to whatever you like, page numbers become irrelevant and it remembers where you are in your reading. Some ereaders also synchronize your reading locations, so you could start reading on your home computer, then go out and resume reading on your phone or tablet right where you left off.