Hello TopoTim
I assume you are a tourist and have purchased a Swiss Travel Pass, not a SwissPass. The SwissPass is a resident product, and you should not mix up the two; otherwise it may lead to misunderstandings as by @FabianBl.
If you purchased the Swiss Travel Pass before creating an account, you cannot add it afterwards to the SBB Mobile app. Simply use the PDF that you have got, store it on your phone, and show it together with your physical passport or government ID card (physical: paper copies or images on smartphones are not accepted).
Kind regards,
Roland