Hello santiago_
Here it is morning 🙂
It is not possible to link the Swiss Travel Pass that you acquired from a third-party reseller. It works only if you had got it from SBB.
In your case, simply use the PDF that you got containing the QR code and your physical passport or government ID card (no paper copies, no images of it on your smartphone).
Swiss trains are free seating, you do not reserve it (with the exception of some PE - panoramic express trains like Glacier Express, Gotthard Panorama Express, or GoldenPass Express).
Simply choose a train, bus, tram etc., hop on, find a free seat, have a ride, and get off where you want to. The Swiss Travel Pass offers travel at will.
More on seat reservations here:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Kind regards,
Roland