Yes, you can watch Pretty Little Liars online for free, as there are a number of different websites that host videos of the show's episodes. Please note, however, that such activity may be illegal.
There are a vast number of online resources available to people in order to watch Pretty Little Liars, or any other show that you could be thinking of watching. One such website is Project Free TV, which is what is termed a 'streaming website.' A streaming website is a site which holds links to a shows episodes on other websites, but does not actually host the videos themselves.
The reason that Project Free TV, as well as other sites, do not actually host the videos themselves is because such activity would constitute a criminal offense, which would be punishable by law in most countries. However, linking to, and streaming from, other websites appears to be in some sort of legal gray area, as the site itself is not actually hosting the video. This is why a number of online resources routinely have links to certain shows, such as Pretty Little Liars, removed and replaced on a regular basis. You should also be aware that watching a television show online on such websites is technically classified as a criminal offense.