When it says that the font is a demo and don’t to the author, what does that means we have to do? I tried to donate, bit it asked for pricer article, and I would like to know what it means? what is an article in this situation?

Usually the demo version has only a basic character map; (alphabet, possibly numbers, some limited punctuation). The full version would be for sale somewhere. It would likely include many accented characters, and other glyphs that are non-essential for basic text use.

Years ago, some font authors released demo versions with a few random glyphs not included, (no lower case k, no capital P, no number 4, etc.), but new fonts like that would not be posted on dafont today; a full alphabet in the character map is necessary for non-dingbat fonts.