Download accelerators are software programs that are supposed to help the user to download large files. When your web browser tries to download something from a website, it opens up a connection to the site's HTTP server and downloads the file through that (single) connection. This is the normal behavior of web browsers.
Download accelerators use a feature of the Web protocol to request a "range" of text. Normally used to resume downloads, this is abused by web accelerators to open very many simultaneous connections and download many parts of one file at the same time. It uses a scarce resource -- open HTTP connections -- in a very selfish, abusive manner and amounts to a denial of service attack against a website. A side like LiberatedManuals, that holds PDF files in a database, needs to retrieve the whole PDF document frmo the database every time a request for a smallf raction of it, is made.
Think of this as you would think of a car driver, who arrived to a gas station, and would use all gas hoses simultaneously to fill his large tank. I am sure that you and other drivers would not appreciate this, nor would the owner of the gas station.
Most of the time, accelerators do not achieve improved download speed for the user, but they do abuse resources of the web servers. They are, therefore, evil.
Liberatedmanuals.Com uses a limit of no more than 10 PDF requests per minute, which is very liberal for regular web users, but prevents abusive downloaders from damaging my infrastructure. If you have a downloader installed, use Save As feature of your browser to save the PDF file on disk in one stream, then view it.