You can't put a PDF in a xfaForm variable. The xfaForm variable can only contain the form data but not the entire PDF.
If you want to use the PDF, you need to use a Document Form variable.
So you should use a 'document' input variable to get the document from the email, then use a SetValue and put that document into the Document Form/object/document property of a 'Document Form' variable. Finally use the Document Form variable in the Assign Task service
Do you have a submit button on you PDF? You need that to be able to submit.
Are you adding the form bridge to the PDF to enable the communication between the submit button on the form and the Workspace interface (FormAugmenterService from Inject Form Bridge service under Process Managment)?
I have added the Process fields in the form, which included the submit button.
I have also used the Form Augmenter Service in Workbench.
The strange part is when the PDF(DocumentForm variable) is viewed in Workspace, the complete/approve/decline buttons appear but I cannot proceed ahead.
For arguments, I used a PDF form, in Workspace, which validates mandatory fields before submission. The same validation kicks off when I click the complete/approve/decline button but it doesn't proceed to the next stage.
I have added an execute script module (as a place holder) in the next step to complete the workflow.