Some are free to educators. Here's a list, in alphabetical order, of some plugins we have used and like . Those marked with asterisks are the ones you should probably try first. Because you could have two or three plugins for one file format, you may not be able to use the plugin you prefer. There is no current standard for the spelling of many terms related to technology.
Pronunciation is also a tossup for some terms, like acronyms.
Plugin is a fairly new word that has recently been used a great deal. The usual pattern in the development of new English words is from a two-word compound, to a hyphenated compound, to one word.