why can I not convert pdf into word file?

I converted in the online, but the word document comes up blank. 

Are you using Acrobat, or the ExportPDF online service?  Either way, what exactly mean "can not"?

This is not a feature of Adobe Reader. You'll need either Acrobat or a subscription to one of the web-based services that Adobe offers (like PDF Pack) to do it.

I cannot even begin to guess unless you tell us at least how are you trying to do the conversion.