As of AEM 6.0 GA release, it is not possible to use any AEM Form that is stored in AEM repository, directly as asset to the Livecycle Assign Task Activity. In order to do that, either you have to copy the Asset in LiveCycle LCA as well or somehow write an intelligent code for example, which passes this PDF form to LiveCycle process as input while invoking the process which then can be further used in Assign Task as document variable.
By the way, if you were talking about Adaptive Form, those are not supported by LiveCycle workspace, currently.