Is there an add-on to auto compress files while uploading into Plone?
As Maulwurfn says, there is no such add-on, but this would be fairly straightforward for an experienced developer to implementing using a custom content type.  You will want to be pretty sure that the specific file types you're hoping to store will actually benefit from compression (many modern file formats already include some compression, and so simply zipping them won't shrink them much).
Also, unless you implement something complex like a client-side Flash uploader with built-in compression, Plone can only compress files after they've been uploaded, not before, so if you're hoping to make uploads quicker for users, rather than to minimize storage space, you're facing a somewhat more difficult challenge.
Is there any add-on which will activate while uploading files into the Plone site automatically? It should compress the files and then upload into the files. These can be image files like CAD drawings or any other types. Irrespective of the file type, beyond a specific size, they should get compressed and stored, rather than manually compressing the files and storing them.I am using plone 4.1. I am aware of the css, javascript files which get compressed, but not of uploaded files. I am also aware of the 'image handling' in the 'Site Setup'