IDM downloads small files (HTML) instead of RAR, ZIP or other large files. Why does it happen?

Probably, you try to download from some sharing site like RapidShare, MegaUpload, FileSonic etc. and IDM is not configured correctly for this site. Please read how to use IDM with sharing sites.

In addition, maybe the link you try to download is not a real download link and you need to open it in your browser first. It may be as well, that in this case, you will see a web page with explanation of the problem. For examle maybe this link requares authentication and you need to login on this site in your browser.