Most of us use word processing software like Microsoft Word, Apple Pages or OpenOffice Writer to create doucments these days. Although we see the words on the page we seldom realise how much more information is included to make the words look like they do: the type of font; size; decorations; etc. This is what I call meta-text information or meta-data.
Often when you copy and paste from one program to another it copies all of the information including meta-data. There are ways to get rid of this, but the simplest is to paste it into something which refuses to accept meta-data like Notepad (Windows) or TextEdit (Mac) and then cut and paste again into the desired program or if you are a corpus linguist just save it as is to use as a plain-text file.
Note: I have heard of problems with MacEdit not saving it correctly for concordancer use. I cannot verify this but I know it works well in Windows witht he method described here and on the linked page.