Why can't I read documents that I have downloaded?

For some reason, I cannot read any text documents that I have downloaded, although the document opens, it is just completely unreadable, just a page full of jargon, number and letters. Whe I open it with Works, its just say Works cannot open file, may be in use by another application, the file format may not be supported by any of the installed converters or the file may be corrupt. I have used a number of programmes to open this downloaded text doc to make it readable without success.