You will need to be able to either dotwalk to the custom table (Federal Onboarding Questionaire) from the HR Case or otherwise query to find the appropriate record in that table based upon the information in the HR Case.

In the attached screenshots, I have the HR PDF Template associated with HR Case. The Address field is using an advanced script which takes the GlideRecord for the HR Case and dotwalks to HR Profile. If I wasn't able to dotwalk from the case to HR Profile I would have needed to query using something on the case (e.g., subject person).