The xfaForm variable is really just a pointer to and XDP in the repository and the data submitted by the user when they hit submit.
That being said the Node Content parameter of the storeContent operation needs a variable of type 'document'. The variable of type 'document' can contain pretty much anything.
If you want to store the data that was submitted by the user you can use a setValue to assign the data of the xfaForm variable to a document variable (ex. /process_data/@myDocVar = /process_data/xfaForm/object/datasets/data). This would store the xml data in Content Space.
If you want to save the entire PDF you have two options. You make sure the form submits the entire PDF, in which case you need to use a different variable type (other than xfaForm) since it can only hold xml. You would use a Document Form variable type.
The other option is to render the PDF on the server using renderPDFForm with the xml data submitted by the user. Once the pdf is created, pass that to the storeContent operation.
Jasmin