Is there a way to do form filling with XPS documents in C#?
Here's how you do it.
First, you create an object (POCO is fine) that contains the form fields you wish to have the user fill in for your form. Next, you create your form as a FixedDocument. It should have Bindings that work against your POCO. Then you create a Window that is set up so that the user can enter values for those fields.
Bind the POCO (plain old CLR object) to the TextBoxes in the Window. After the user has entered the form data, take your now filled-in object and set it as the DataContext of the FixedDocument. Create an XpsWriter and Write your FixedDocument to your output (file, printer, etc).
I'd give you code, but since I'm actually doing this (or something relatively similar) for work I don't think I can. I can tell you that the hardest part is dealing with the XPS document. You can judge relative to your experiences there.