Encyclopedia is a book or series of books that contain general information about many topics or areas. The word encyclopedia has been in use for at least 500 years. It was used in print for the first time as Encyclopaedia or Knowledge of the World of Disciplines in 1559.

Often confused with a dictionary, an encyclopedia is inherently different from a dictionary. Its contents have more details and offer deeper insights into the topic. A dictionary merely gives definition to words, while an encyclopedia explores topics more deeply with illustrations, maps, tables and photographs. While most of them are arranged alphabetically, some encyclopedias are organized by categories or cross-referenced articles.

The first encyclopedia appeared in the first century BC. It had 37 volumes and covered subjects from anthropology, human physiology to painting, agriculture, and pharmacology. Then came an era of religious overtones, the first Christian encyclopedia in 560 AD, followed by first Muslim encyclopedia.

In 1403, the Yongle Encyclopedia was published in China as the longest encyclopedia ever, 11,000 handwritten volumes were created. Incidentally 400 volumes of those times are still available.

However, the 20th century was the era of Encyclopedia Britannica as the standard for encyclopedia works. Many encyclopedias were published in CD-ROM also, like Microsoft's Encarta.

In 21st century encyclopedias are now accessible online. Undoubtedly an advantage of online encyclopedia is the easy access to an ocean of knowledge, however its greatest benefit is that you can download only those pages that you need, not the whole book. Since these pages are in PDF format, you can even find a way to edit them if you want.

To edit PDF pages, the trick is to convert them into editable format first. You can google search to find a site that converts PDF documents into editable format. You are likely to get many sites that will offer to convert your PDF online. Some sites are free and some may charge a small fee, say 50 cents a page DIY or so. Make sure you select the site that does high quality conversions for you, such sites may have a fee but every dollar you pay will be worth its value many times.

It goes without saying that there will always be a tendency to select free sites because they are free, but that's not what you should do, instead checkout first if a site is good and is safe for you to use.

Thats because Free sites are not considered safe anymore. A McAfee study found that searching for free stuff online increases your chance of landing on a site littered with malware. According to the study, by inserting the search term "free" with a popular keyword, there is 300% increase in the chances of hitting a malicious site.

So make sure you do the right thing, and since you have to make a decision 'to pay or not to pay' you now know which one is safe.