Current Pages, Numbers, and Keynote documents are zip files where the .zip has been replaced with the respective document extension. This is also true of Microsoft .docx, .xlsx, and .pptx documents. Zipping them again won't buy you anything.
Dropbox officially supports Keynote presentations providing they do not exceed 288 MB. Google Drive/Docs has been known to automatically unzip Apple documents, and it may be that those cllcking on your Keynote link are experiencing this issue. This is something that you should take up with Dropbox, as they may have changed something recently that has caused this problem — without it necessarily being a Google issue at all.
There is nothing Apple can do about this problem.